A new lawsuit in California, accusing Hyundai of running misleading fuel-economy ads for its Elantra sedan, points out why consumers need to beware of mpg claims in car advertising.

The lawsuit alleges that Hyundai has run ads for the Elantra focusing on its fuel economy and noting that the EPA rates the car at 40 mpg-without including a mandatory disclaimer that that 40 mpg is only a highway fuel economy rating. The lawsuit was filed by a California law firm along with Consumer Watchdog, an advocacy group.

We noted last year that lots of automakers are now focusing on their cars’ highway fuel economy claims, rather than city or overall mileage as they used to. Perhaps they just want to show higher numbers. And we went so far as to test three such small cars and found that they don’t always live up to these highway mpg claims. (See our video of the Chevrolet Cruze Eco, Ford Focus SFE, and Honda Civic HF).

Even when they do, we think few drivers spend all their time on a highway, and that overall mpg ratings are more relevant for most consumers.

And unlike the EPA, we conduct our own fuel economy test using real cars on real roads. (EPA uses a treadmill and mathematical adjustment factors.) While our overall mpg figures are similar for most cars, our city estimates are often much lower. And while the Elantra is rated at 40 mpg in the EPA highway test, it couldn’t quite match that number in our real world testing. It returned an overall average of 29 mpg, and narrowly missed 40 mpg on the highway, returning a still-respectable 39 mpg.

It just goes to show, you need to be careful what you believe in advertising. And that’s why we’re here. Consumer Reports was founded on a  mission to give consumers accurate information to counteract misleading ads. And as long as there is advertising, that’s what we’ll continue to do.