The agencies will accept public comment for 60 days before choosing one of the two stickers.

Gina McCarthy, E.P.A. assistant administrator for air and radiation, said in a conference call that the grading system would compare cars against a full range of models, “not just S.U.V.’s against S.U.V.’s.” She said there would be no failing marks.

The highest grade, A+, with fuel economy rated as equivalent to 117 miles per gallon and up, would be reserved in a sample rating for “zero emission” electric cars. Plug-in hybrid electric cars (which get rated at the equivalent of 59 to 116 miles per gallon) would get an A grade, and some conventional hybrids, like the Toyota Prius and Ford Fusion, would get an A-. Other hybrids, like the Nissan Altima, Ford Escape and Toyota Camry, would receive B+ grades.

If the grading system existed now, under assumptions developed by the agencies, 306 small cars from model year 2010 would receive a B, only eight S.U.V.’s would receive a B+ (68 would get a C), and the highest grade for a van would be a C+, officials said.

Image This sticker would give letter grades for fuel economy and greenhouse gas emissions.

Dave McCurdy, chief executive of the Alliance of Automobile Manufacturers, an industry group, said the rating system “falls short because it is imbued with schoolyard memories of passing and failing.”

In an e-mail, Gloria Bergquist, a vice president with the group, wrote, “Note to media: Your spokesperson tried to give the proposal a D but was told not to do so by her boss.”

Some environmental and consumer groups reacted more positively. Dan Becker, director of the Safe Climate Campaign, which is part of the Center for Auto Safety, a consumer advocacy group in Washington, said in an e-mail: “Over all it’s a B.”