"CAFE miles per gallon ratings and EPA window sticker mpg ratings were originally generated simultaneously through government lab testing," said Edmunds.com Director of Vehicle Testing Dan Edmunds. "Window sticker ratings have been down-rated twice in the last 25 years to make them more realistic for consumers, while CAFE mpg methods remained the same. So, with each EPA revision, EPA window sticker mpg and CAFE mpg drifted further apart."
Indeed, the difference between the two is significant -- big enough to drive an SUV through in some cases.
For example, a vehicle that scores an EPA rating combined city/highway of 29 miles per gallon actually contributes 39 mpg to its manufacturer's CAFE average.