How to Understand Automakers’ Gas Mileage Claims

The most important concern consumers have in shopping for hybrid cars is fuel economy, or gas mileage. When researching gas mileage of hybrid cars, keep in mind the following:

If you own a hybrid car, you can generally expect your fuel economy to be 10–20% lower than the numbers indicated on the sticker. For instance, a hybrid that the manufacturer claims will get 50 miles to the gallon will probably get closer to 40–45 miles to the gallon. This variation in mileage results from several factors, such as the newness of the car, the condition of the road, and the driver’s style.

Somewhat lower mileage isn’t a reason to dismiss hybrid cars—in fact it’s rare for any car’s everyday fuel economy to match the manufacturer’s claims. Also, dealer claims actually tend to be more accurate for hybrid cars than for conventional cars.

Automakers are required by law to measure and report the gas mileage of every model of car they manufacture. But the U.S. government–mandated test procedures used to determine fuel economy use pre-production prototypes and don’t simulate modern driving conditions. For example, the average test speed for highway driving is 48 mph, resulting in gas mileage estimates that are inaccurately high for both hybrid and conventional vehicles. (The EPA has announced that it intends to update the way it calculates fuel economy—the changes are expected to be introduced with 2008 models.)

Even so, the manufacturer’s gas mileage numbers are not worthless information. Prospective buyers should consider them relatively accurate: if a manufacturer claims that a hybrid gets 50 mpg while its conventional equivalent gets 35 mpg, you can be confident that the hybrid will get about 40% better gas mileage than the conventional model.