In February, the EPA announced its revised fuel economy testing procedures. Effective beginning in the 2008 model year, the revised tests include two additional evaluations that assess vehicle performance while driving at higher speeds and while using air conditioning. The new procedures will result in lower mileage ratings for all vehicles, including hybrids. The Toyota Prius, for example, will see its combined fuel economy rating drop from 55 MPG to 46 MPG, a reduction of 16%. Ratings of other hybrids, such as the Camry Hybrid, fall as well, but by a slightly smaller percentage (13%).

Will potential hybrid buyers be turned off by lower EPA ratings? It remains to be seen. While roughly 80% of U.S. households know something about hybrids, only a small number of them have actually shopped for one. That means a lot of people haven’t paid much attention to the fuel economy ratings for a specific hybrid model in a given year. If these consumers shop for hybrids in the future, they may never know that the vehicles had higher mileage ratings in the past. So while the changes to EPA ratings may be a big deal to those of us who watch hybrids closely, many consumers may never notice.

There is some concern, however, about the symbolic value of high MPG. Many consumers don’t think in terms of percentage changes in fuel economy—they think just about the MPG number. Reducing the Prius’ MPG from 55 to 46 seems like a more drastic change than reducing the average car’s MPG from 25 to 21. In fact, both represent a change of about 16%. So while the new ratings only slightly alter the fuel efficiency advantage of hybrid models relative to conventional vehicles, they disproportionately lop off more miles per gallon from hybrid models. For some buyers, this could rub some of the sheen off the hybrid halo.