Low gas prices mean more SUVs -- and that's led the EPA to lower its forecast for overall fuel economy.

When it unveiled its new rules in 2011, the EPA expected an average of 54.5 miles per gallon by 2025. That's the average for all vehicles.

On Monday, it lowered the forecast to between 50 and 52.6 miles per gallon -- unless gas prices head higher.

Related: Coolest cars for $18,000

Consumers shouldn't expect to get such good gas mileage in real life, though.

The fuel economy rules are part of the Corporate Average Fuel Economy program, which is run by the EPA and Department of Transportation.

The fuel economy estimates used for CAFE regulations are different from the numbers seen on the window stickers at dealers

Related: Seven super tiny SUVs

CAFE fuel economy numbers are used only for regulatory purposes, and are usually about 20% higher than those more realistic numbers. So a CAFE fuel economy figure of 52.6 mpg figure translates to about 42 mpg on the road.