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've tested got far fewer miles per gallon than their window stickers claim," said Consumer Reports Automotive Editor Rik Paul.
For example, the EPA says the Ford C-MAX hybrid 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"We think the problem is that the EPA ratings are based on outdated tests that don't reflect real-world driving conditions for hybrids," said Paul.
One of the highway-driving tests the EPA performs is 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," said Paul.
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 engines, so they burn more gas than they do in the EPA tests," said Paul.
Consumer Reports has discussed its findings with the EPA and the agency says it's considering updating its tests.
Consumer Reports fuel-economy tests also show 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.