Published: Jul 18, 2013 5:50 PM EDT
Updated: Jul 18, 2013 6:20 PM EDT

FORT MYERS, Fla. - When you're shopping for a new car, the window sticker says how many miles per gallon you can expect to get. Those numbers are an estimate based on tests developed by the Environmental Protection Agency. But when Consumer Reports ran its own fuel economy tests on 315 cars, the results for many hybrids were surprising.

"Hybrids tend to be very fuel-efficient. But many of those we tested got far fewer miles per gallon than their window stickers claim," said Rik Paul with Consumer Reports.

For example, the Ford C-Max hybrid, the EPA says it gets 47 miles per gallon overall. But in Consumer Reports' tests, it was 37 miles per gallon overall, still good, but about 21 percent less than the EPA estimate.

"We think the problem is that the EPA ratings are based on outdated tests that don't reflect real-world driving conditions for hybrids," Paul said.

Take highway driving, one of the tests the EPA performs on a "dynamometer." It tests cars at simulated speeds that average just 48 miles per hour with a lot of stop and go.

"Hybrids do well in those driving conditions. They can often operate in electric mode without burning any gas," Paul said.

But Consumer Reports tests highway mileage on a highway, at a steady 65 miles per hour. Technicians install a fuel meter to measure the amount of gas burned.

"In those conditions, hybrids are constantly running their gas engine, so they burn more gas than they do in the EPA tests," Paul said.

Consumer Reports has discussed its findings with the EPA, and the agency says it's considering updating its tests.