Why the 2025 fuel economy standard means 40 mpg, not 54.5

Toyota Prius hybrid

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Cars sold in 2025 will have to average 54.5 miles per gallon. If that sounds impossibly high considering cars on the highway are a mix of efficient Toyota Priuses and thirsty Chevrolet Silverados, and everything in between, don’t worry quite so much. The real fuel economy number is much lower, on the order of 40 mpg or so. In other words, fleets that meet the 54.5 mpg government standard are actually returning a little over 40 real-world miles per gallon. The difference lies in the way the government calculates fuel economy. There’s lab testing behind it, but the numbers are inflated.

What the government is talking about for the 54.5 mpg number is CAFE, or the corporate average fuel economy. CAFE is based on repeatable laboratory testing but the tests don’t necessarily measure real-world conditions. The original CAFE standard, in 1978, was 18.0 mpg for passenger cars. Currently it’s 30.2 mpg for passenger cars, 24.1 mpg for light trucks, an average of 27.5 mpg. That’s the standard that goes to 35.5 mpg in 2016 up to 54.5 mpg in 2025. In case you haven’t figured it out yet, the average new car sold today doesn’t actually get 30 mpg — as in when you drive 100 miles starting with a full gas tank, you need 3.3 gallons to refill the tank.

CAFE numbers were so out of whack that back in 2008 the EPA added a fudge factor to CAFE calculations to bring cars and light trucks more or less in line with real-world driving conditions. The adjusted numbers are what’s on the window sticker in the dealer showroom: city mileage, highway mileage, and combined driving (45% city, 55% highway) mileage. The CAFE number lives on for the purpose of calculating averages, gas guzzler penalties for Lamborghini and Ferrari, and the 2025 standards. Real-world or window sticker mileage is about 25% lower than CAFE mileage. So 54.5 mpg is 40-41 mpg. If the fudge factor is 20%, then it might be 44-45 mpg.

EPA dyno testing

EPA dyno testing

Why did fuel economy testing go so wrong? The original tests didn’t call for the use of air conditioning. They didn’t include cold weather driving. And because cars that were accelerated hard tended to shoot right off the roller wheels in the lab floor, gentler acceleration was called for, even when better equipment became available. The old EPA tests were effectively goals for hypermilers before there was hypermiling (maximizing fuel economy with leisurely starts, low cruising speeds, gentle braking, and the heck with the long line of cars stuck behind you).

Politics plays a role as well in fuel economy ratings. Remember the ethanol craze of the past decade? America’s corn farmers convinced the government that turning corn into ethanol and creating a fuel called E85 was our path to energy independence. Normal gasoline is about 90% gasoline, 10% ethanol, with the ethanol component making the fuel burn cleaner. E85 is 85% ethanol, it corrodes the fuel lines of cars not intended for E85, you get worse fuel economy per gallon burned, and it’s not widely available. But if the automaker builds an E85-capable, or flex fuel vehicle, they get a CAFE fuel economy credit regardless of whether the vehicle ever uses E85. The flex-fuel car gets worse mileage; the government credits it with improved mileage.


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  • Anonymous

    Good article.  As is normal, we are being hoodwinked with the real numbers.
    The government should require manufacturers to quote combined mileage in ads, not the inflated highway number.  And you are right on with ethanol, one of the biggest scams of the last 20 years on the public.E15 is coming, and I have two cars that will suffer damage if I use E15 in them.  The gas companies are supposed to label the pumps if E15 is being pumped, but we shall see.

  • Anonymous

    What about the cell fuel engines or hydrogen?

  • Michael Griffin

    If the politics and OEMs would get out of the way we could see innovations like this:


  • Anonymous


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