Why 62 mpg doesn’t really mean 62 mpg

As wrangling over the next round of federal fuel economy regulations filters out to the public, one number will get a lot of attention.

62 mpg.

Under the most aggressive proposal, requiring 6 percent annual improvement in mpg for the 2017-25 model years, corporate average fuel economy would hit 62 mpg.

To get a sense of scale, remember that automakers just started a five-year regimen to get CAFE to 35.5 mpg. So your average consumer who hears about 62 mpg will look at his or her current car, which maybe gets 20 mpg in daily use, and think, “Man, we’re gonna be driving $50,000 carbon-fiber, electric-drive shoeboxes with all the comfort and charm of an airline economy seat.”

Well, maybe not. For one thing, automakers are pushing for a 3 percent annual increase, with a 2025 target of 47 mpg. And the rules won’t be set until July 2012. So there’s a lot of posturing, rhetoric and horse-trading to look forward to.

But here’s another reason: 62 mpg doesn’t really mean 62 mpg.

At least, not in terms of what U.S. drivers would see on window stickers or experience when driving 2025 vehicles. Under a 62 mpg CAFE rule, real-world fuel economy would be just under 50 mpg.

And if CAFE were 47 mpg, the real-world number would be about 38 mpg.

To know why, you have to understand what the CAFE number is -- a sales-weighted average of the mpg ratings for vehicles produced in a given year. Vehicle mpg ratings are based on lab tests using a dynamometer, a sort of treadmill for cars.

Dynamometer testing produces an artificial number, but it does provide controlled conditions. No wind, no rain. And it allows for precise test protocols. For instance, the federal city-driving test cycle lasts 1,874 seconds, with an average speed of 21.1 mph, and has cold- and hot-start segments of 505 seconds each.

That’s where CAFE mpg numbers come from. But -- here’s the curveball -- those numbers don’t appear on window stickers.

In an effort to get closer to real-world fuel economy, CAFE numbers are reduced by 20 to 25 percent, depending on the type of vehicle. So a car that scores 35 mpg on the laboratory test will have a window-sticker rating of 28 mpg.

Does the system confuse consumers? Probably. Does it produce tidy, replicable CAFE tests? Probably.

Anyway, just remember: 62 doesn’t mean 62.