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Why US Mileage Ratings Are So Inaccurate

Posted by timothy on from the if-you're-wrong-enough-ways-at-once dept.

Why does a car rated for 47mpg fall so far short? The Houston Chronicle features an article on just why EPA gas estimates can be so different from real-world drivers' experience at the pump (or in looking at the dashboard display), in particular for hybrid cars. From the article: "A geometric average of the FTP-75 and HFET results (with city driving weighted at 55 percent and highway driving weighted at 45 percent) produces a vehicle's CAFE fuel economy, which is then incorporated into a manufacturer's corporate average. CAFE is measured using these tests to the present day. In fact, this methodology will be 50 years old when it's used to gauge compliance with the forthcoming 54.5-mpg CAFE requirements in 2025. That kind of continuity is admirable in baseball, but not in transportation. These tests are irrelevant to contemporary real-world driving. For example, the maximum acceleration on either test is 3.3 mph per second. At that rate, it takes more than 18 seconds to hit 60 mph. Even in the horsepower-deprived 1970s, most people were driving harder than that. And the 60-mph maximum speed on the highway test does not accord with the 75-mph truth of today's interstate traffic."

24 of 374 comments (clear)

  1. consistency more important by mondovoja · · Score: 5, Insightful

    Whether those numbers represent a real world mix of driving accurately really doesn't matter all that much, since fuel economy for other driving styles strongly correlates with fuel economy for the conditions that are actually measured. Long term consistency, on the other hand, matters a great deal for car buyers and for evaluating progress on reducing emissions and consumption.

    1. Re:consistency more important by swalve · · Score: 4, Insightful

      Yes, exactly. The CAFE ratings aren't meant to tell you what your personal MPG is going to be, they are meant to tell you how cars of a specific model year compare to each other. If you do 10% better in one car, you'll probably also do 10% better in the other one.

    2. Re:consistency more important by realityimpaired · · Score: 5, Interesting

      Yes, exactly. The CAFE ratings aren't meant to tell you what your personal MPG is going to be, they are meant to tell you how cars of a specific model year compare to each other. If you do 10% better in one car, you'll probably also do 10% better in the other one.

      That may be so, but miles per gallon is a misleading measurement on which to base the "10% better" calculation. 30MPG to 33MPG is *not* 10% better efficiency. In fact, the two are so close that it's within the margin of error for most of us, and can easily be outweighed by simply getting a bad dice roll with the traffic lights.

      If they switched to a burn rate measurement, like L/100km (that the rest of the world uses), or even Gal./100mi, then you actually could do the math in your head for how much more or less efficient the vehicle is. MPG isn't a linear scale, but L/100km is. As a result, the higher the MPG, the less actual benefit you get: 50MPG to 75MPG isn't even close to a 50% improvement in fuel efficiency (it's actually only about 25%), and the disparity between reality and perception only gets worse as you get higher.

    3. Re:consistency more important by GrumpySteen · · Score: 3, Insightful

      If they switched to a burn rate measurement, like L/100km (that the rest of the world uses), or even Gal./100mi, then you actually could do the math in your head for how much more or less efficient the vehicle is

      Let's take a closer look at your point:

      2L/100km is obviously twice as efficient as 4L/100km since 4 is twice as large as 2. Easy enough to do in your head.

      50km/L is obviously twice as efficient as 25km/L since 50 is twice as large as 25. Just as easy since it's the exact same math.

      The problem with your idea is that you read that a 10 mpg increase in fuel efficiency doesn't represent the same percentage of increase when it's applied to different starting mpg figures (i.e. from 30mpg it's a 33% increase, but it's a 50% increase if you start with 20mpg). And that's true, but you're assuming it affects all calculations using mpg figures. The L/km measure behaves in exactly the same non-linear fashion. A 1L/100km increase in efficiency from from 3L/100km is a 33% increase but it's a 50% increase from 2L/100km. Once again, the math is exactly the same.

      tl;dr L/km isn't particularly better than mpg. You just suck at doing math that you haven't practiced, so you think it's harder.

    4. Re:consistency more important by nabsltd · · Score: 5, Insightful

      I think that over a certain speed, fuel economy goes way down.

      The smaller the engine, the more you will see this effect. This is because it has to work much harder (relatively) to move the car faster.

      For a larger engine on the same car, you generally get worse gas mileage at lower speeds because the idle fuel usage is higher. But, as you go faster, the larger engine doesn't work much harder, so mileage doesn't drop as much. After a certain speed, a larger engine will be more efficient on the same car.

      This is why turbochargers are now all the rage. You get the advantage of the lower idle consumption of the smaller engine, while still having the acceleration you need for emergencies and the better mileage at higher speeds.

  2. Not only the milage ratings are false by Anonymous Coward · · Score: 5, Interesting

    The biggest reason that real-world fuel economy is so different is that the testing is done with a specific "standard" fuel that does not contain any ethanol or other "oxygenator for cleaner burning fuel". The stochiometric ratio required for proper catalitic converter operation on modern cars is maintained by the oxygen sensor adjusting the amount of fuel injected into the engine - too much oxygen in the exhaust gas, add fuel to decrease; too little oxygen, decrease the amount of fuel. This is a closed-loop system that does not take into account fuels that have additional "oxygenators" added - it only cares about the oxygen in the exhaust gas. Add oxygen from fuel additives, reduce oxygen in the exhaust gas by adding more fuel, reduce mileage. "Clean burning fuels" with additional oxygenators is one of the biggest government-mandated ripoffs ever devised. The "testing" done to prove the "value" of oxygenated fuels is done with a single-cylinder carbureted engine in a test lab, with no emission control systems. In the "bench" testing, a specific amount of fuel is burned with the oxygen in the air, and the resulting exhaust gases analyzed for hydrocarbon emissions. Add an "oxygenated" fuel, burn the same specific amount metered at the same air-fuel ratio, and TADA, look, it burns cleaner! Of course it does - there is now additional oxygen in the exhaust gas! But in the real world, the emission systems on a modern car sees the extra oxygen and adds more fuel to the engine to "correct" the air-fuel ratio and reduce the oxygen level in the output gas. They don't tell that part to congress or the consumer, so the use of "oxygenated" fuel is mandated by the law at both federal and state levels - and so 4.) Profit!

    And the milage you get on the road does not match the testing...

    note: I designed and manufactured fuel control computers for a while, so I know a littile about how things work.

  3. Re:Simple explanation by Anonymous Coward · · Score: 4, Informative

    American highway speed limits vary depending on the state and location. It's usually 65 but inside a large city it is often 55. In the Texas desert it can be 85.

  4. Re:Choice of average by swalve · · Score: 4, Informative

    When you are calculating the average of ratios (miles per gallon) and the denominator is the thing that changes, you have to use the geometric mean. If they used liters per 100km, then they would use the regular average.

  5. Your mileage may vary... by dtjohnson · · Score: 5, Insightful

    No one cares what the testing procedure is as long as everyone does the same test and it's repeatable. The purpose of the test is to provide a method for consumers to compare different models with respect to their fuel economy, not to provide a precise prediction of exactly what the buyer's fuel economy will be. Everyone drives differently. People warm their car up in the driveway, fill it up with heavy weight, carry lots of passengers, do a lot of long-distance driving, tow trailers, drive up and down hills, ride their brakes, accelerate briskly to beat their neighbor, drive at high altitudes, drive in cold weather, or whatever. Even more significantly, the energy content of 'gasoline' varies widely depending on how much ethanol it has (more is less) and what its boiling point range is. Just do the same test and do it in a way that someone else could repeat the test the same way and get the same result. That's all we need rather it's a 50 year old test or not.

    1. Re:Your mileage may vary... by AmiMoJo · · Score: 4, Insightful

      The problem is that manufacturers tune their cars to do well in these tests at the expense of efficiency in more realistic conditions. Nobody accelerates at 3mph/s but cars are optimized for that because that's the test that is administered. If they made the test more realistic everyone would benefit.

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  6. Re:Simple explanation by houghi · · Score: 4, Informative

    http://en.wikipedia.org/wiki/Speed_limits_in_the_United_States
    The highest speed limits are generally 75 mph (121 km/h) in western states and 70 mph (113 km/h) in eastern states. A few states, mainly in the Northeast Megalopolis, have 65 mph (105 km/h) limits, and Hawaii only has 60 mph (97 km/h) maximum limits. A small portion of the Texas and Utah road networks have higher limits.

    So if you live in a 60MPH state, it is accurate. If you live in a 75MPH then it isn't.

    So they should have different information for different states. And obviously for city driving as well.

    --
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  7. Re:Units wrong by xaxa · · Score: 5, Informative

    For example, the maximum acceleration on either test is 3.3 mph per second.

    It's hard to take a paper seriously when it gets the units of measure wrong.

    What's the problem? That *is* an acceleration.

    (The SI measure is ms^-2, metres per second squared, or metres per second per second. 3.3 (miles/hour)/second = 0.44704 m s^-2.)

  8. Re:Simple explanation by kwark · · Score: 4, Interesting

    The European tests are also flawed, they might be more realistic but the "mileage" is still not applicable to real world situations. The tests are highly optimized, there is almost no way to get these results as an ordinary driver.

    There as a consumer program on TV a couple of weeks ago, people were complaining they were only getting 16km/l instead of the advertised 25 km/l for a certain car. This was after driving instructions/coaching from the importer. The conclusion was something like:
    Every car is tested in the same way, highly optimized. You will not get these results in real life, but you can use the results to compare cars, a 25 km/l car will be more efficient than a 20 km/l car of the same fuel type for the same driver.

    BTW I am able to almost reach the manufactures mileage in my car, but it means I have to drive really slow, stay of the throttle (0-100 km/h in 20s), look ahead/anticipate to avoid breaking/acceleration, drive under the max speed limit, don't drive in the city, don't drive during rush hours, make sure the car is empty (not carrying unnecessary weight). But realistically this will almost never happens.

  9. OK by JustOK · · Score: 5, Funny

    Could someone explain this with a car analogy?

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  10. In any case... by BrokenHalo · · Score: 5, Funny

    I was under the impression that the standard unit for fuel consumption in the US is furlongs per hogshead.

    1. Re:In any case... by JustOK · · Score: 4, Funny

      It was updated, now it's LoC / firearm

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  11. Fusion Hybrid Owner by OneOver137 · · Score: 3, Interesting

    I'm own the much maligned 2013 Fusion Hybrid, and my current tank is averaging about 44 mpg. My work route currently averages between 43 and 50 mpg.

    My driving conditions are a mix of heavy suburban traffic and stretch of 25-55 mph interstate, with speeds averaging 15-20 mph during rush our. The terrain is rolling hills, with a delta of about 200 feet.

    On a warm (T >70 degree), dry day with no wind and little traffic, the car will easily get the 47 mpg.

    Temperature has a large impact on the mpg. The same example above in 25 degree weather will net about 36-38 mpg, consistent with the reporting done over the winter. Obviously, cold starts and running the defroster has a big effect, and the electric traction motor eats away at the battery much quicker at lower temps.

    Rain will cut the mpg on my work route to about 43 mpg, and the extra drag is very noticeable. A headwind has the same effect. Tailwinds are fun though, and it kinda feels like sailing when the ICE is off.

    Cruising at 55-60 mph on the highway, in no traffic on a warm, dry, and windless day, I can get the 47 mpg.

    A quick temperature and mpg plot (assuming dry, windless conditions) looks like:

    (T deg F, mpg): (25, 36), (30, 38), (40, 40), (50, 43), (60, 45+), (70, 47+), (80, 45).

    There is some roll-off at the higher temps because you have the A/C running.

    Driver style has a huge impact on observed mileage, and this cannot be stated enough. My wife is your typical, jackrabbit starting, bumper riding, race-to-red driver. Her mpg is far worse than mine. I doubt she's ever seen 40 mpg. A trip that I can do at 45 mpg, she'll get 36 mpg. I've tried to coach her on the basics of hybrid driving, but she just doesn't get it. I imagine a lot of people are the same way. You either "get" how to drive a hybrid, or you don't.

  12. Re:The Testing Process is Flawed by Ironhandx · · Score: 4, Informative

    Consumer reports tends to be a bit sensational. They do get put on a treadmill but the EPA numbers are also based on a circuit of real road driving. 3 tests are done and the average of those tests are then used as the EPA rating.

    Additionally theres no such thing as a frictionless treadmill, and the treadmill they use is actually able to adjust its load to simulate real world resistances.

    There are plenty of real reasons to bash the EPA ratings, there was no need for consumer reports to exaggerate and make shit up.

  13. Re:Drive conservatively! by FlatEric521 · · Score: 3, Insightful

    I agree. On my car with a built-in MPG screen, if I drive well, I get about 26.5 to 27 MPG on a car that's rated 23/31. Driving through town poorly, my wife gets about 23. Driving straight through to Vegas (from SoCal), I've gotten 32. And this was on the old "inaccurate" EPA scale. Seems pretty accurate to me.

    I would recommend you consider double checking your car's trip computer calculation against the tripometer and gas pump readout method of manually calculating miles per gallon. I bought a Hyundai with a trip computer that includes Avg. MPG as one of its readouts. Prior to owning that car I had gotten into the habit of resetting the tripometer on my car at every fuel up after writing down the miles from the tripometer and gallons from the pump readout (then just divide the miles travelled vs gallons to refuel the tank). I kept up that habit after getting the Hyundai and found that the manual calculation method consistently reports 2-4 MPG lower than the trip computer. If your trip computer is anything as optimistic as mine, then you may actually be getting less than you think.

  14. Why mileage ratings are inaccurate by emaname · · Score: 3, Insightful

    I worked for just over 2 years in a wind tunnel for a company that manufactured cooling equipment (eg, radiators, oil coolers, A/C condensers and evaporators). We tested products for a variety of manufacturers which meant a wide variety of equipment; ie, compressors, farm tractors, semi tractors, passenger cars, and on one occasion a small city bus to be used in Miami, Florida.

    We had a reputation for maintaining a very stable, controlled environment (air flow, heat load, dynamometer load, and positioning of thermocouples for sampling temperatures) and consequently consistent test results.

    Now in the interest of full disclosure, this was in the early 70's. But at that time, that's also where the manufacturer's typically got their mileage estimates.

    I think this might also be the era from where we get the expression "Your Mileage May Vary" (aka YMMV). I think they included this disclaimer in car ads in an attempt to comply with the "truth in advertising" laws (remember those?).

    Clearly nobody can drive a vehicle in a manner as controlled as that.

    So if the manufacturers are still getting their mileage results from a wind tunnel test, forget it. You'll never match those results especially if you live in a large metropolitan area (where it's not uncommon to sit idling in traffic) or you live in a mountainous area or where you have really cold weather.

    There are several really good comments here with additional insight as to why mileage can vary drastically from the manufacturer's estimate; type of fuel mix, for one.

    So remember, when you're buying a car and read those mileage estimates, YMMV.

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  15. Re:Drive conservatively! by danbert8 · · Score: 4, Insightful

    Incorrect. The correct driving lane is the rightmost lane you can occupy. If you are going 100mph you should still be in the right lane if you aren't passing. Idiots like you cruising along in the middle lane are why I end up passing you on the right. If you are in the correct lane you will NEVER be passed on the right.

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  16. Re:Speeds by beelsebob · · Score: 3, Insightful

    Where I live the posted limits are 75 mph. Very few people here drive that slow unless the radar detector is going off or there is a cop actually in view.

    The reason I know this is because I *do* drive the posted limit, and I am *constantly* being passed.

    Arguing semantics But actually, that isn't valid reasoning. Even if you're constantly being passed, that's not a proof that the number of people passing you is greater than the number of people matching your speed exactly.

  17. Re:Efficiency by icebike · · Score: 3, Informative

    Speaking of power in the gas tank, it seems that all these tests are conducted with straight gas, but all you can find at the pump these days is 10% ethanol, which immediately cuts your mph by 3 to 4 mph.

    Very few new car advertising even mentions the difference.

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  18. Re: Efficiency by icebike · · Score: 4, Informative

    Entire article is false, EPA changed the testing in 2008. Since 2008 they test mpg up to 80mph and accelerate at 8.5 miles per second, roughly 0-60 in 7 second

    So I'm guessing you never bothered to read the article. (I know, what was I thinking).

    The new standard is FULLY addressed in the article. Go read it.

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