Fuel Efficiency Numbers Overstate MPG More For Cars With Small Engines 403
whoever57 writes: All official numbers for fuel economy in the EU typically overstate the miles-per-gallon figure that drivers can expect to achieve in typical driving. A recent study confirmed this once again. However, what the study also found was that MPG figures are more unrealistic for cars with smaller engines than for cars with larger engines. Actual MPG figures achieved based on typical drives for cars with small engines could be as much as 36% under the official number, while those cars with 3-liter engines would typically achieve 15% less than the official figure.
These discrepancies need to be accounted for if we're going to be serious about regulating fuel efficiency. But then, we should be using gallons-per-mile instead of miles-per-gallon, too.
metric you insensitive clod! (Score:5, Informative)
No. You should be using litres per kilometer. Especially so when talking about the EU.
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As much as I like to bash American units they aren't really the issue here.
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As much as I like to bash American units they aren't really the issue here.
How is "miles/gallon" wronger compared to "gallons/mile" than "gallons/mile" to "l/km"?
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How is "miles/gallon" wronger compared to "gallons/mile" than "gallons/mile" to "l/km"?
One problem with miles per gallon is that there is one kind of mile, but two kinds of gallons. US gallons are smaller than UK gallons, so if I tell you my miles per gallon, you need to know where I am.
In practice, you report "litres per 100 kilometre", because litres per kilometres should be a tiny number, somewhere between 0.04 and 0.1 for most cars. Americans and Brits should feel free to do litres per 100 miles.
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The reason it's considered inferior is because it's inverted from what you really care about - what you care about is "how much fuel will it take me to get n miles".
No, what I really care about is, "can I make it to the next fuel stop with what I have in the tank." Which is not a problem in most of Europe, but is very much a problem in large parts of the USA.
And unlike the manufacturers' economy claims, I use the number on a regular basis instead of just when I'm planning to buy a car.
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It's rarely an issue even in the states, but having recently driven across Utah and Idaho, there were times when I did have to pay close attention to the gauges. (Also that crazy strip of touristy Oregon coast where even with a town of 20k and a bazillion tourists, the nearest gas station was 20 miles away, but let's not dwell on that.)
The thing is, my car (a 2010 model) still doesn't actually tell me how many gallons are in the tank. I've just got a readout with a dozen or so dots that slowly disappear as
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That's what a trip odometer ca
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I agree with you in spirit - and in fact, do the same thing - but let's admit what a piss-poor solution that sounds like.
Instead of having a moderately accurate measurement of how much fuel our cars have remaining, we find it more reliable to make all sorts of assumptions about driving conditions and weather and long-term averages and whether or not we "topped off" that last 25 cents on our last fill... And then use our subsequent driving distance to guess ho
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My 93 Cadillac gave me gallons of fuel in tank, average mpg, estimated real time mpg, estimated miles on remaining fuel and my 2006 Buick though it may not show fuel in gallons still gives me average mpg and miles on remaining fuel.
If you really want those features buy a different car.
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The reason it's considered inferior is because it's inverted from what you really care about
This is especially apparent when computing averages. If ten people drive SUVs that get 10 miles/gallon, then the average MPG is 10. If one of them switches to a super hybrid that gets 110 MPG, the average goes up to 20 MPG. That is twice as good, right? Wrong, total consumption has gone down less than 10%. So using average MPG is pretty stupid, but that is exactly what the government does with the CAFE standards.
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No, it isn't [wikipedia.org]. The CAFE standards traditionally used the weighted harmonic mean of the mpg values, which gives exactly the same result as the weighted arithmetic mean of the economy expressed in gallons per mile. There are some other quirks- dual fuel vehicles are treated much more favorably than they probably ought to be, for instance- and the standards were recently changed to give bigger vehicles a break. But the larger point is that the
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So why does the UK (which is small, crowded and has expensive fuel) use the same units?
(Well, the gallons different, but dimensionally it's still the reciprocal of an area)
whether metric or not, distance per volume rulz! (Score:3)
For many of us in the USA, you're assumption about what we care about is bullshit wrong.
I live in Portland, Oregon. If I'm going north or south, I'm sometimes interested in mpg, but it isn't a big thing. Never very far from an Interstate highway in those directions.
But if I'm going northeast or southwest, mpg is critical, because there are too many back roads in the Washington and Oregon outback where running out of gas would put one in serious danger, and not be just a bit of an inconvenience. Having to
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The places I want to go are NEVER on the direct route between A and B. Oregon sunstones are more than 70 miles from the nearest gas station, and the last 30 miles are gravel. That's 140 miles of poor gas milage with no chance for a fill up.
Back roads to trail heads at Paulina Lake, into the Strawberry Mountains, or the fossil beds are even worse.
Once you get out of Mama's basement, there is a wonderful world out there to explore. Using MPG rather than some other fuel consumption measure makes those explorations just a little bit easier.
Quite apart from the random ad hominem, saying that we should use MPG because it's marginally more useful for a tiny share of total trips taken in the US, and only in those cases for the small portion of cars that don't have distance to empty available, and for the drivers of those cars who can't be bothered to fill up at a gas station before venturing out on a 150 mile round trip, just doesn't make sense. Somewhere, there might be someone who has once made a trip in a car where the odometer had been custo
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That's good, because this is a British news source talking about a British problem :)
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Re:metric you insensitive clod! (Score:5, Funny)
"The metric system is the tool of the devil! My car gets forty rods to the hogshead and that's the way I likes it."
Re:metric you insensitive clod! (Score:5, Funny)
Your car can't even make it between gas stations, sadly.
40 rods = 0.125 miles
1 hogshead = 63 (US) gallons
0.125 miles / 64 gallons
0.001953125 / gallon
10.3125 feet/gallon
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It's even funnier now you've explained it all in intricate detail.
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Even simpler (Score:5, Informative)
would it not be better to simplify a volume divided by a length to an area. Gallons/mile is best represented in represented in hectates or furlongs^2.
conversion factor (Score:4, Insightful)
1 US gallon / mile = 0.00364583333 sq inches
conversion factor (Score:5, Insightful)
Now lets try the same using the metric system:
1 litre / kilometre = 1 sq millimetre
That is another win for the metric system in my book.
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Express both using metric units and tell me, if you have to travel 1285km and your vehicle uses 3.7l/100km, or, rather, gets 27.027km/l, how much fuel do you need?
Do the math for both LPK and KPL, show your work. Here, I'll do it for you:
Using LPK (3l/100km):
1285km * 3.7l = 4754.5
4754.5 / 100km = 47.545l
OR
1285km / 100km = 12.85
12.85 * 3.7l = 47.545
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1 US gallon / mile = 0.00364583333 sq inches
--
Some drink at the fountain of knowledge. Others just gargle.
You, Sir, are just gargling. And you've dribbled on your shirt.
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My favorite unit PSI (Score:3)
My personal favorite is how Americans measure pressure (such as in tires): pounds per square inch. It is so bizarre, it is beautiful...
The "pounds" are pounds of force (lbf), of course, but I doubt, an average person (be he American or European) can articulate the difference between mass and weight...
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Up until just a few years ago, the ultimate measure of fuel economy in the UK was:
miles/liter/stone/cubic meter
So I wouldn't gripe about US ANSI units too much ;-)
sPh
Haven't been to the UK since road signs were officially changed to km, but I understand most UKians still think of distances in miles.
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Presumably Europe uses litres per 100 kilometres. At least that's what we use in Canada.
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I was just going to mention that "gallons/mile" instead of "miles/gallon" struck me as fairly similar to standard vs. metric, i.e., mostly a matter of preference in daily life.
It's already a simple mathematical operation to transform from mpg to gpm, and with the wall of numbers he throws at us which I feel that a car owner is only going to actually do when they're in the market for a new car, the annoyance of transitioning over outweighs the benefits. And I'm definitely against further translation by divid
Gallons per mile? (Score:2)
In marketing, bigger numbers are usually better, except for the price. This is why we use MPG and why they put big numbers on the speedometer even though that 4 Cylinder would never make it to 120 MPH.
Windows 7 becomes Windows 8 becomes Windows 8.1. Boeing 727, 737, 747, 757, 767, 777, 787.... Airbus 320, 321, 330, 340, 350, 380... Ford F-150, F-250, F-350. Each increase is supposed to represent the product getting bigger and better.
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Windows 7 becomes Windows 8 becomes Windows 8.1.
How does your theory explain that same series but going backwards from 7?
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Sorry, given this is shashdot, I should have allowed for some production 4 cylinder car to make it past 120 MPH. However, I assure the GEO Metro economy 4 cylinder isn't going to bury the pointer on it's speedometer without going down hill with a strong tail wind.
Remind me not to ride with you.....
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Interesting, the speedometer on my 4-cyl only goes to 85MPH (not that the car can go that fast in it's current condition, there is a reason it's parked until I have the time to repair it).
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This is why we use MPG and why they put big numbers on the speedometer even though that 4 Cylinder would never make it to 120 MPH.
Considering how long Indy racers ran with four-bangers ...
Or for that matter, my Subaru with its four-cylinder Boxer is basically an updated version of the car that holds a long list of speed records for distances like 50,000 km -- at sustained average speeds of over 135 mph.
The real reason auto manufacturers put silly speedo ranges on is to keep the most common highway speeds in the upper quadrant of the dial, for quick reading and thus faster times getting your eyes back on the road. And, yes, I've worke
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Lotus Elise is a 1.4l four cylinder engine and does 150mph.
Its the TEST! (Score:3)
Well DUH! (Score:5, Insightful)
If you try to push a small engine to drive like a larger one, you'll be accelerating harder, therefore using more fuel than under normal acceleration.
In similar manner, some years ago, I had a Grand Cherokee that my wife couldn't get more than 11mpg out of, while I could do 17mpg.
We have different driving styles. She is a leadfoot, while I drive like I have an uncooked egg between my foot and the gas pedal.
Perhaps this article might better be titled "Want better gas mileage? Don't drive like a gashole."
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It tells you exactly why in the article. It's the way people drive them.
If you try to push a small engine to drive like a larger one, you'll be accelerating harder, therefore using more fuel than under normal acceleration.
In similar manner, some years ago, I had a Grand Cherokee that my wife couldn't get more than 11mpg out of, while I could do 17mpg.
We have different driving styles. She is a leadfoot, while I drive like I have an uncooked egg between my foot and the gas pedal.
Perhaps this article might better be titled "Want better gas mileage? Don't drive like a gashole."
Yep, your right foot is the determining factor. I am able to achieve the manufacturer's fuel consumption rating in my car if I accelerate slowly and obey the speed limit. I did it as an experiment, as I usually drive more like your wife.
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You are correct in that often it is the nut behind the wheel that has the greatest affect on mileage. If I drive like a reasonable person in my daily drive I get in the 35-37 mpg range (summer time) but when I wa
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Perhaps this article might better be titled "Want better gas mileage? Don't drive like a gashole."
Duh. When you have a tiny engine, you have to thrash the crap out of it to get anywhere. Which is probably the point of this article... the tests are unrealistic for small engines because they don't drive it the way a real driver would.
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It tells you exactly why in the article. It's the way people drive them.
Doubly-so when we're talking about the vehicles in question in the article. Small displacement cars in the EU are, almost entirely, manual transmission vehicles. This means that you can precisely shift at 1500 RPM on the dynamometer test (which doesn't have any hills, traffic, or risk of death if you stall out), crawl your way up to speed, and get excellent l/100km results. This would be completely suicidal on an Autobahn or Motorway.
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There's also the European preference for small high-revvers combined with the disdain for automatic transmissions. Yes, up through about 1990 a well-driven manual could provide better fuel economy. Today's computer-controlled automatics are more efficient than human shifters, and that's before any fancy radar-driven predictive shifting is brought into play.
sPh
Note that I am saying nothing about personal driving enjoyment preferences or ability to play boy racer, just fuel economy
Don't even think it (Score:2)
Well it was interesting until they started talking about everything being more efficient at 55 mph. At that point, I was ready to give the author a boot party. Do not EVEN start that sh*t again.
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It's true, though. For most cars, fuel economy declines as speeds climb past 55-60mph (wind resistance being non-linear). You're trading off fuel for time - get there faster, but use more fuel. We should let people make that tradeoff for themselves, however. Just price fuel appropriately (including the externalities of climate, military expenditures, etc.), and let drivers decide.
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Well it was interesting until they started talking about everything being more efficient at 55 mph. At that point, I was ready to give the author a boot party. Do not EVEN start that sh*t again.
Well, physics is physics. That seems to be the sweet spot, generally speaking. However, fuel economy is not my primary concern when I drive. I drive a fast car and enjoy driving it. The feeling of being pushed back in my seat and feeling the tires grip in corners is worth a few extra dollars to me.
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There is also the problem with gear boxes. When you are in the top gear (or over drive) at 55 MPH, when you drive the speed limit at 70+ MPH you are 15 MPH into the top gear, thus increasing the revs just to drive the speed limit.
The funny thing is for the longest time we heard you never need more than a 4 speed automatic. Then the EPA changed the MPG ratings to include 65 MPH and now we got 6 speed automatics. Then they added 70 MPH to the test and now we started getting 7-9 speed automatics. Funny how
Better Data Shouldn't Be That Hard (Score:2)
Practically every card on the road today has a feature which calculates MPG (or L/100KM) historically. Just add a data field in the car's computer that keeps the historical number, even if the one on the dash is reset, and download it from a % of cars at their annual inspection. Won't help for new models, but will, over a couple of years, develop a very robust data set saying "the Ford Model XYZ tested at X MPG, but real world MPG are Y." Not flawless, naturally, since a different set of drivers for each
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Not going to happen unless it's legislated. You can't trust the calculation given by the cars, and the automakers absolutely do not want every car to track this since they leave themselves open to lawsuits when the numbers don't meet whatever was advertised. For example, the readout on my car gives L/100km but it's rounded to the whole number! So it's useless even if it was accurate because the precision is gone.
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Not going to happen unless it's legislated. You can't trust the calculation given by the cars, and the automakers absolutely do not want every car to track this since they leave themselves open to lawsuits when the numbers don't meet whatever was advertised. For example, the readout on my car gives L/100km but it's rounded to the whole number! So it's useless even if it was accurate because the precision is gone.
The real issue here is that those numbers depend entirely on how the end user drives the car anyway. I used to own a Subaru Legacy GT LTD, with a turbo. The stock 0-60 time was 4.9 seconds. It could take off in a hurry. When I felt like being a bit on the sporty side, I would often get an abysmal 14-15MPG. If I were feeling a bit more conservative, I could easily get 24MPG in the same conditions. If I drove especially fast on the highway, I would get 21MPG. If I drove conservatively, I could get 28MP
Numbers based on optimal conditions (Score:2)
Hell, they would do it downhill if they could getaway with it.
As for 'more for small cars', if you remove 30 lbs from a 6,000 lb vehicle, that is 0.5%, but if you do the same for a 3,000 lb vehicle, it is 1.0%. So yeah, optimization works better for a smaller car.
Driving style (Score:3)
Probably most of it comes down to driving style. People who are used to older cars with bigger engines will probably think a new model with a small engine is gutless and will floor the accelerator to make it go faster. Anecdotally, I drive a 2005 Civic Hybrid, which was originally rated for ~46 MPG with the "less realistic" measure EPA used back then. I've driven it 170k miles now and that is in fact its lifetime average - it has two trip odometers and I never reset one of them. However back when these were still pretty new I read reports of people who complained about getting only ~33 MPG out of an identical car. The only reasonable explanation is that they were flooring it between stoplights and generally ignoring the instantaneous and cumulative MPG display the car gives you.
Basically people are impatient and don't know how to drive efficiently. It took me a few months to really get into the groove with mine and I still have to make sure I've got plenty of room to pass on the highway, but it's certainly doable.
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Yransportation efficiency (Score:2)
Most cars are only carrying the driver, and doing speeds of less than 120 kilometres per hour.
In the rest of the world you don't need a multiton SUV for those uses.
A 1 litre normally aspirated 4 cylinder should be enough.
You can also get more MPG by using proper size gallons. (4.54 litres)
Short Trip mileage (Score:2)
Obligatory metric troll (Score:2)
We *should* be using L/100km, like everyone else.
And taxing fuel at a higher rate instead of this CAFE silliness. But that's never going to happen because if we know one thing about economics in America it's that all taxes are always bad.
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What possible benefit is there to taxing fuel, other than to hand more money to the government to waste?
Oh, yeah, I forgot, it lets you force people into small cars they don't want, or force poor people onto buses.
Why do you hate the poor? What did they ever do to you?
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What possible benefit is there to taxing fuel, other than to hand more money to the government to waste?
Oh, yeah, I forgot, it lets you force people into small cars they don't want, or force poor people onto buses.
Why do you hate the poor? What did they ever do to you?
The rationale for taxing fuel is to capture the externalities (pollution, climate, military costs) of using that fuel. The point about the regressiveness of the gas tax is valid, so we should raise the gas tax, but add a refundable credit to income taxes for it, to remove the regressiveness.
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All you have, less the costs you impose on others through pollution, use of state services, etc.
Original link has more data (Score:4, Informative)
The study is by Emission Analytics, and here is the original link [emissionsanalytics.com] (as opposed to TFA from The Telegraph).
Note some misleading elements from TFA: they show only the three smaller classes for UK cars, seemingly indicating that small cars are the worst gas guzzlers, whereas cars with higher engine sizes are actually much worse according to the original study (see the graph [emissionsanalytics.com]). So the lesson is: still buy a small car, just not a very small one for best fuel efficiency.
Another interesting bit that is not in TFA is that the data for US cars is different: there, cars between 1 and 3 liters in volume (I assume this is the large majority of the car pool) have less than half the mileage. Also, the smallest US cars are actually the most efficient of any class, even though their efficiency is below UK average.
Why Overstated? (Score:2)
1. Carmarketers like good gas mileage figures; they're good for sales.
2. The specifications for the test are gamed to provide a bigger benefit for underpowered cars which tend to get better mileage anyway. The test include acceleration at a rate *that depends on the car's power* (percent of full-throttle). which has the big-engine (more powerful) cars zipping around the virtual course at higher speeds.
Remember, lobbyists write or co-write most of our laws and regulations.
Top Gear had an interesting experiment (Score:2)
Some time ago I had a big old V8 car and I could pretty much halve my mileage simply by being only somewhat more aggressive. City driving would also send that car's mileage into a tailspin. The rated mileage was around 23/18Mpg but I would sa
What we generally do - in this country.... (Score:5, Insightful)
In that case, knowing distance per unit of fuel is more important than fuel per unit of distance.
In Europe, where distances are SIGNIFCANTLY shorter it is much more interesting to worry about the cost of the trip, especially when public transport options are close competitors in price. In this case the unit of fuel per unit of distance makes a much easier comparison.
Opposite from my experience (Score:2)
Yeah, yeah, one data and all that.
My 2010 Hyundai Elantra claims it will get 29/40. The first time I took it on an extended (highway) trip, I got exactly their 40 mpg figure (actually a fraction above).
As to local driving, I filled up yesterday and the calculation gave me 32.22 mpg though I don't drive what one would consider true city driving such as in New York or LA, more a hybrid of stopping and starting with some distances in between. That is comparable to my usual number with has been as high as 35
The Bureaucrat Effect (Score:3)
This is what happens when uneducated, uninformed Peter-Principle bureaucrats dictate fuel efficiency standards without checking with real engineers to find out if it's even possible.
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I like it when people quote their fuel efficiency numbers and then I find out they were getting the measurements from some readout on their dash. It takes quite a bit of actual logging and averaging to produce real fuel efficiency numbers and I doubt that many people do it at all.
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Most dash readouts that I know of display both instantaneous MPG and average. They can still be inaccurate but the average should only be off by a factor as modern cars know exactly how much fuel they're allowing into the engine.
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But many of them stop calculating your MPG when your motor is running but you aren't moving, which provide very bad estimate of your true efficiency..
In mine, the average mpg display goes down continuously if I'm stuck in a traffic jam. It doesn't display current mpg until you reach 15mph or so, but the average mpg display does got updated.
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I regularly fill up at the same station, so I do the math myself when I'm bored (I reset trip odometer each tank so I know how many miles I've gone).
Shouldn't modern fuel injection computers know how much fuel is being injected? It seems like it should be trivial for the car to tell me my MPG correctly.
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Curious as to why the fuel economy readouts on a modern car would be inaccurate. The computer has fuel flow readings down to about .001 ml and precise wheel rotation readings 6/sec from the ABS system. Unless the owner puts tires of a non-standard diameter on the car what would cause the inaccuracy?
sPh
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The only way to get more reliable distance measurement is by GPS, so by all means go for it. However, it is easy to get a far more accurate fuel use number by using the actual volume of fuel you paid for, and it's the same for all vehicles since we all use the same calibrated pumps. Ideally, one wouldn't rely on any of the onboard instruments to calculate these numbers, but we're pretty sure the odometers aren't completely fraked up. I can't say the same about the fuel flow meter.
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To be fair, Britain has orders of magnitude fewer stop signs than the US (we use give ways at almost every junction, stops are reserved only for blind, or otherwise dangerous ones). We also have significantly safer roads than the US. So yes, it does appear that in large part, stop signs are there to annoy skilful drivers like him.
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What you wrote is not what he said.
He said that rolling through them saves fuel, not that there was no reason to ever have a stop sign, or that they were annoying at all.
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I drive a Focus ST, and during extensive MPG testing, I've seen this first hand. A turbo 2.0L engine, capable of much speed and acceleration, and I usually maintain about a 24-25 MPG average, which is the rating for all city driving, but I don't drive it gently most of the time.
When driving a car like that, you should be measuring fuel in smiles per gallon.
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Quoting MPG is useless unless you clarify whether you're quoting US-MPG or IMPERIAL-MPG... 25 IMPERIAL-MPG is 20 US-MPG.. A somewhat less impressive number. This is slashdot; we don't have the benefit of location context.
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I always thought that the best way to increase fuel economy would be to have a MPG indicator on the dash of every vehicle. If you could tie it into a local average fuel price to give cost per mile, even better. I know when I had it on my car, I was more inclined to preserve my money.
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I find that pretty much all cars are capable of reaching the MPG ratings on their windows stickers... the problem is, you have to drive under certain circumstances to reach those
Bingo. In the real world you have to keep up with traffic, stop/start all the time and generally drive in a way that doesn't inconvenience other drivers too much. Everyone hates hypermilers but that's what you have to do to get the advertised numbers.
depends on the manufacturer (Score:2)
In my '06 VW TDI I was pulling 42+mpg (trip odometer and gas receipts) consistently with the cruise set at 65 and 79mph (highway/interstate) with little in-town travel. And I'm the kind of guy that, "drives it like he stole it". Even now with my new commute being ~15 mintues of stop-and-go traffic and ~10 minutes of blasting around country corners and hard accel/braking I still manage 34-36 mpg.
The '06 VW Golf TDI was rated at 31 city, 40 highway, 34 combined:
http://www.fueleconomy.gov/feg... [fueleconomy.gov]
One day I bumpe
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I've found that I usually exceed EPA estimates on 6-cyl vehicles (I'm using the highway numbers to compare since that is most of my driving) (test cases: 95 Chevy Blazer, Automatic, 4WD got ~20.5 MPG vs EPA 19; 99 Ford Mustang, Automatic, I'm seeing ~26.5 vs 25, 98 Mustang, Automatic, 25 vs 26), and have mixed results with 4-cyl vehicles (95 Saturn SL, Automatic, 37 vs 33; 88 Mustang, Automatic, 20 vs 25; 99 Chevy Malibu, 28 vs 28). I don't have enough data on 8-cyl vehicles to compare (I've only had one o
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Probably stuffed full of ethanol, to keep corn farmers employed. Might as well put water in your tank.
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I can only assume that the tanker driver accidentally dumped the "good stuff" super unleaded into the ordinary unleaded tank, because last weekend I purposely bought the more expensive super unleaded (again I'm not saying which brand) and achieved exactly the same mpg on the way home. If this is consistent, it's actually worth buying the more expensive grade of fuel to get the extra mpg.
No, super unleaded (higher octane fuel) will not get you higher milage. The difference is only how much you can compress the fuel before it explodes. This is important in high compression ratio engines (e.g. ones with turbo chargers), but completely irrelevant in normal car engines. In fact, most normal car engines will get slightly less far on higher octane fuel, due to it containing slightly less energy.
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This article is talking about the UK. A 3 litre engine there is virtually unheard of. A 5 litre engine would be reserved for a tractor or heavy goods vehicle. Heck, even our top racing series (Formula 1) uses 1.6 litre turbos.
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America is different. My neighbor's old 1970 Cadillac used to have a 500 cubic inch motor. Math says 8.2 litres or slightly more than 1 litre per cylinder. Of course the fuel tank was about 80 litres. And you would fill it once a week if you drove it to work.