The U.S. EPA allows automakers to self-test their mpg ratings, there’s a degree of an honor system under current rules, but following corrections by Ford and Hyundai/Kia, the federal agency is tightening its procedures.

Specifically, how automakers prepare vehicles for road load values to measure rolling resistance and aerodynamic drag are being revised from a 50 mph coast-down on a straight and flat test track to 70 mph.

The coast-down load values are used to calibrate the automakers’ dynamomenters when they measure vehicles according to the EPA test cycle.

This stricter procedure will also see more audits of automakers and potential fines to keep automakers honest and accurate.

In 2012 Hyundai and Kia were found to have miscalculated the coast-down testing of vehicles leading them to estimate higher than realistic mpg ratings on window stickers including hybrid models. They’ve since been hit with a $350 million fine and have apologized for their part in the error.

Another loophole yet open was capitalized upon by Ford which rated its C-Max Hybrid the same as the more aerodynamic Fusion Hybrid because they shared the same powertrain.

This loophole is still open, and the EPA is looking to close it. Ford has downgraded its models twice in the past couple of years.

The EPA’s Chris Grundler, director of the Office of Transportation Air Quality said the government remains concerned how automakers are grouping vehicles.

And of course, even if everyone does fastidiously follow the rules, the old adage “your mileage may vary” remains true, and probably more so for hybrids.