People who care the most about getting great gas mileage may be disappointed to learn what Consumer Reports' latest tests have uncovered.

Those tests show that hybrids - while some of the most fuel-efficient cars on the road - are all too often coming up short.

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," said Consumer Reports' Rik Paul. "But, many of those tested got far fewer miles per gallon than their window stickers claim."

For example,  with the Ford C-Max hybrid, the EPA found ti gets 47 mpgs overall. In Consumer Reports' tests, it was 37 mpgs overall.  That's still good, but 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"We think the problem is that the EPA ratings are based on outdated tests that don't reflect real-world driving conditions for hybrids," said Paul.

For highway driving, the EPA tests cards on a dynonometer, using simulated speeds that average 48 miles an 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 instaledl 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," said Paul.

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