You look at the window sticker and it reads “35 mpg,” so you think to yourself – why, that’s great, perfect, a nice car with great gas mileage. Sold!

Not so fast. The numbers that more Americans are considering seriously when it comes to the purchase of a vehicle may not be very accurate, and, in fact, they may be changing. According to Autobytel’s own analysis of recent vehicles and a report from Consumer Reports, the EPA’s rated miles per gallon can be much greater than what you will get on the road, especially when it comes to hybrid vehicles. According to Consumer Reports, they found that the EPA’s estimates ran 35 – 50 percent higher than the vehicle’s actual fuel efficiency. Hybrids averaged 19 miles per gallon less than their EPA ratings, according to the publication, and ratings were too low for 90 percent of the vehicles tested. Autobytel’s own road tests support these findings. For example, a road test in the Toyota Prius consistently registered a combined fuel economy of 45, compared to its listed EPA rating of 60/51, and the Ford Escape Hybrid registered just 25 combined city and highway miles per gallon – though the EPA fuel economy is listed at 31/27. Worse yet is the new Toyota Highlander, whose “real world” miles per gallon came in at 19.8, compared to a stickered EPA estimate of 31/27. The same holds true for traditional powertrains – though the difference is generally not quite so dramatic.

Part of the reason for the difference lays in the laboratory approach the EPA uses to test its vehicles. According to fueleconomy.gov, “the vehicle's drive wheels are placed on a machine called a dynamometer that simulates the driving environment—much like an exercise bike simulates cycling. The energy required to move the rollers can be adjusted to account for aerodynamic forces and the vehicle's weight. On the dynamometer, a professional driver runs the vehicle through a standardized driving routine, or schedule, which simulates “typical” trips in the city or on the highway.”

It promises to get even loopier, when the EPA revises its ratings beginning with the 2008 model year, with another adjustment in 2011. All of a sudden, buyers used to seeing one thing will get a lower, and more accurate rating. It makes fuel economy rating more realistic, especially when it comes to hybrids, and that’s a good thing for car buyers. For example, according to Toyota, the Prius hybrid will be adjusted downward, 20 percent for city driving estimates and 11.8 percent for highway driving – a decrease that brings the Prius down to earth in terms of fuel economy, with a 48 city and 45 highway rating. This is a stunning adjustment, especially when you consider that many hybrids claim to be notably fuel efficient in city driving circumstances. But then again, “your mileage may vary.” Below, we’ve taken a calculator and come up with possible mpg adjustments for several notable hybrid cars. Note that the Toyota Prius is an estimate provided by Toyota. For other vehicles, we use the highest percentage as reported by the EPA: 20 percent for city, and 15 percent for highway estimates.

Possible MPG adjustment (City/Hwy):
Toyota Prius
Old: 60/51
New: 48/45

Honda Civic Hybrid
Old: 49/51
New: 39/43

Ford Escape Hybrid
Old: 36/31
New: 29/26

Photos courtesy of the automakers