WASHINGTON, DC (WUSA) -- When you're shopping for a new car, the window sticker displays how many miles per gallon you can expect.
It's an estimated number based on tests developed by the Environmental Protection Agency.
But Consumer Reports ran its own fuel economy tests on 315 vehicles, and 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," says Rik Paul.
For example, the EPA says the Ford C-Max Hybrid gets 47 miles per gallon.
But in the independent consumer groups'test, it only got 37 miles per gallon.
It's still good, but about 21 percent less than the EPA estimate.
Paul says, "We think the problem is that the EPA ratings are based on outdated tests that don't reflect real-world driving conditions for hybrids."
For instance, highway driving. It's one test the EPA performs on a dynamometer. Cars are tested at simulated speeds that average 48 miles per hour with a lot of stop and go.
"Hybrids do well in those driving conditions. They can often operated in electric mode without burning gas," Paul says.
Consumer Reports test for highway mileage on a highway at a steady 65 miles per hour.
The techs 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.
The consumer organization spoke to theEnvironmental Protection Agency about its findings.
The agency says it is considering updating its fuel economy tests.
The consumer group also says fuel economy tests of cars with small turbocharged engines often do not deliver on the mileage promised.
Those include the Buick Encore, Ford Fusion and Nissan Juke.
They fell short of the EPA estimates by 10 percent or more.