Outdated Tests Skew CAFE Gas Mileage Ratings: 48-MPH Freeways?

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Highway traffic

Highway traffic

It's widely accepted in the auto industry that all manufacturers game the test cycles used to establish emissions and gas-mileage ratings.

Ask auto engineers about the topic, and they tend to get a faraway look and go silent.

But the test cycles established in 1978 and used ever since are far from the reality of U.S. driving in 2011. This means the results have to be manipulated using various "adjustment factors" to have any semblance of real-world relevance.

One minute to 50 mph

Take acceleration, for instance. How fast does your car go from 0 to 60 mph?

If it's a standard family sedan or crossover utility vehicle, the answer is probably 9 to 13 seconds, depending on its engine and how aggressive a driver you are.

The EPA, on the other hand, thinks it takes more than a minute to accelerate from 0 to ... 50 mph.

Top speed: 60 mph

Los Angeles Smog

Los Angeles Smog

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And the top speed on the highway test cycle is 60 mph, which may have made sense when the national limit was 55 mph, but is ludicrously low today. When's the last time you saw average highway speeds of just 48 mph?

Consider that data gathered several years ago from several hundred cars in the Los Angeles Basin found that when freeway traffic was flowing smoothly (it doesn't always), the average vehicle traveled at a speed of more than 80 mph. That's an average vehicle, mind you, not the fastest one.

It gets worse. Three decades ago, air conditioning was still a high-end option, rather than the virtually standard feature it is today on North American vehicles. But neither AC or heaters are switched on in any test cycle.

mustang window sticker

mustang window sticker

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While the EPA is in the process of converting over to new, more representative test cycles to get the ratings you see on new-car window stickers, that's not the case for the NHTSA, which specifies the CAFE ratings carmakers must achieve.

The tests to determine Corporate Average Fuel Economy (CAFE) ratings--which will rise to 54.5 mpg by 2025--rely on the old cycles, and hence are increasingly divorced from reality.

Unrealistic hybrids ...

Historically, the EPA had to make additional adjustments for new technologies, including hybrid-electric vehicles that can run on electricity alone under some circumstances.

During the last decade, the test cycles were so unaggressive that hybrids spent far more time in electric mode on the tests than in the real world, producing wildly high ratings that few owners came anywhere close to.

Public complaints finally forced the EPA to revise its adjustment factors in late 2007, lowering hybrid fuel economy to levels closer to what owners actually achieved.

... but penalties for diesels?

2010 Volkswagen Jetta TDI Cup Street Edition

2010 Volkswagen Jetta TDI Cup Street Edition

Enlarge Photo

On the other hand, owners of diesel-engined cars have complained for years that the EPA ratings are too pessimistic, and that owners of Volkswagen Jetta TDIs see far higher real-world mileage than the EPA number indicate.

Finally, the outmoded procedures actually penalized some new technologies. Consider the start-stop feature, which simply turns off the car's engine when it's stopped, then switches it back on when the driver starts to lift a foot off the brake.

According to Robert Davis, senior vice president at Mazda's U.S. arm, the company would like to introduce its iStop technology to North America as it rolls out its new line of ultra-efficient SkyActiv engines and transmissions.

2012 Mazda3 SkyActiv 2.0

2012 Mazda3 SkyActiv 2.0

Enlarge Photo

But thus far, it won't, because the gain on the CAFE cycles would only be 0.1 or 0.2 mpg, not enough to justify a cost of a few hundred dollars--despite fuel savings of up to 10 percent in dense urban stop-and-go traffic. The city cycle is mostly quick stops, like those for stop signs, with only a few lengthy stops as you would get at a traffic signal.

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Comments (10)
  1. i regularly exceed epa fuel economy numbers on the high way but there is no way you can match the city number. but since 99% of my daily drive is all highway i can live with it.

  2. The value in the EPA numbers is that they provide a consistent test method that cannot be generated from users or journalists.

    That said, it would be bad if the test does not give adequate credit to technologies like start/stop or diesel.

    Just for comparison, I looked at UK data using the NEDC for a Diesel (VW Jetta), Hybrid (Prius), and ICE (Honda Accord).

    For the Highway, the EPA numbers are lower by
    27% (Diesel)
    22% (Hybrid)
    13% (ICE)
    So if you believe NEDC, both hybrids and diesels are punished by EPA and the ICE rating is pretty close.

    On the other hand, if you look at that city data
    22% (Diesel)
    13% (Hybrid)
    -12% (ICE)
    In this case, if you believe NEDC, EPA punishes mostly diesels and promotes ICE cars.

  3. John: Just to confirm, I trust you corrected for the British gallon being 25 percent larger than the U.S. gallon? These results roughly match what I've seen, in that the Euro cycles seem to produce somewhat higher fuel efficiency than the U.S. numbers after they've been adjusted for window-sticker use.

  4. That is right. I used the liters/100 km numbers as some journalist educated me to do. I can send the data, but it is difficult to post here.

  5. I also learned that it is difficult to compare USA and Europe cars because the USA typically gets larger engines than in Europe. But I am sure you already knew that. I used the Accord because it seemed to have the same engine in USA and Europe.

  6. But in the end, what is clear is that Toyota Prius MPG is better in all testing (EPA, NEDC, city, hwy) and this is a story that needs to be told more than diesel possibly being cheated out of a few percentage points.

    Even if you use the NEDC and city numbers, the Prius is about 50% better than the Jetta diesel (4.0 vs 6.1 l/100km). EPA number suggest the Prius is better still (4.6 vs 7.8 l/100km).

  7. The prius is a ripoff. I did the math comparing a prius to say a an elantra. after adding in all costs (mostly finance charges) the math says the average person driving 12k a year would require just a hair over.... get this ... 100 YEARS to break even buying the prius.

    100 YEARS. car won't last 20. even ME driving 40k a year would require 38 YEARS to break even. JUST TO BREAK EVEN.

    the prius is a joke. I get 58-62mpg in my Metro. a freaking tin can $7k car NEW in 1994 car.

    how about putting that prius drive train in a no frills economy car for $12k. THEN you might have something.

    50mpg is a JOKE too. anything less than 80mpg typical in a hybrid vehicle and its by definition a "RIP OFF"

  8. I say slow the heck down. 45mph is my overall average and I do not go faster than 55mph PERIOD. don't like it? are you going to send me a check for fuel? if so I will go any speed you want upto the posted limit.

    I drive a geo metro. even with this filthy dirty polluting expensive Ethanol Laced gasoline I can get 58-62mpg average in the summer on my 54 mile to work commute (NO highway all mixed)

    and that is nothing but easy on the gas and slow down and time my lights. no turning off the engine and coasting. (I can hit 70mpg doing that)

    if its one lane or no one can pass I do the PSL. if you can pass or there are more lanes I do 5 below the PSL I max at 50-55 Unless drafting. Don't like it? let me know when your ready to pay my fuel bill.

  9. ALSO I want to enlighten people. Fuel Economy means NOTHING. absolutely nothing. the reason is pretty simple. Our national average in the USA is 20mpg.

    I drive a 50-60mpg metro. Does that save me money driving this car? trick question.

    answer is NO. I don't save money driving a 50mpg car because it gets 50mpg. I save money because YOU DON'T

    the real issue here is not MPG but CPM. Cost per mile.

    the price of fuel is based on the CPM and our usage. so I save money because I am OUTSIDE the curve.

    if you raised the national fuel economy average from 20mpg to 40mpg tonight by friday the price of gasoline would DOUBLE. it would have to or gas stations would go out of business.

    your CPM would stay the same.

    the only answer is to not play.

  10. I've owned 12 vehicles since 1976 (including my 1976 Honda Civic 5 speed which got 56 mpg) and I have beat the EPA fuel economy rating with all of them, usually by 10 to 20%. Only one was a hybrid, but you don't need a hybrid to recover the kinetic energy of a moving vehicle. Coasting to a stop works just as well. Brakes are highly overrated.

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