Your fuel mileage is lower than you think.

Granted, that assumes you do not fastidiously monitor your own fuel mileage — that instead you take the EPA’s fuel mileage estimates at their word.

Turns out, the EPA calculates fuel economy with “straight” (100 percent) gasoline. However, in the consumer market a blend of 10 percent ethanol, E10, is nearly universal. Jonathan Welsh of The Wall Street Journal explains:

Fuel economy decreases by about 2% for vehicles running on E10, so a car rated at 25 miles per gallon will actually travel about 24.5 miles.

Okay, this decrease is peanuts if you’re a lead-foot. And the EPA just started including air conditioning with this year’s estimates. Nevertheless, even with perfect driving habits, the fuel economy of your brand new Prius will never match the sticker’s claims.