The MPG crapshoot

For model year 2008, the Environmental Protection Agency has revised its test procedure for determing vehicle fuel economy, making it somewhat closer to real-world driving and perhaps somewhat closer to what real-world drivers actually achieve. In general, I applaud this action: if we’re going to do this sort of thing, we ought to at least try to do it accurately.

The EPA now has an online tool whereby you can look up your pre-2008 car (if you already have a 2008 car, I’m impressed) and see how it would rate under the 2008 test. I duly punched in Gwendolyn’s numbers, and was presented with this:

Old: city 20, highway 28, combined 23.
New: city 17, highway 25, combined 20.

The “combined” figure is 55 percent city, 45 percent highway. I note that EPA lists this car as running on “Regular Gasoline”; Nissan specifies premium. I’d expect it might get poorer mileage on the lower-octane stuff.

My own figures so far: city 21 (over 5000 miles), highway 28 (900). Pretty close to the old numbers, and way better than the new ones.

I went back to my previous car, on which I had over 55,000 miles of data, and here’s the chart:

Old: city 22, highway 28, combined 24.
New: city 19, highway 25, combined 22.

Actual numbers obtained: city 23, highway 30.

I’d like to think that the reason why I do at least as well as and sometimes better than EPA’s numbers is because I’m such a spectacularly good driver, but I don’t believe that any more than you do. What I think is happening here is that the cars I have driven were not specifically engineered to get good numbers on the test: there were no characteristics intended to exploit the test conditions. For example: Chevrolet’s “skip shift” on manual-transmission Corvettes would, um, encourage a 1-4 upshift, leaving the car in a tall fourth gear for most of the test, enough for GM to avoid the dreaded gas-guzzler tax.

Still, this doesn’t explain everything. Sandy, my late Mazda 626, had only average mileage ratings and below-average acceleration for her class. However, I drove the living whee out of her and still got better-than-EPA numbers. Gwendolyn weighs 13 percent more and packs nearly 75 percent more horsepower (227 versus 130); then again, she doesn’t have to work so hard.

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