Official release of requirements will come toward end of Bush presidency

WASHINGTON - The next generation of new cars and trucks will need to meet a fleet average of 31.6 miles per gallon by 2015, the Bush administration proposed Tuesday, seeking more fuel-efficient vehicles in the face of high gasoline prices and concerns over global warming. Transportation Secretary Mary Peters outlined the plan on Earth Day, setting a schedule that was more aggressive than initially expected by the auto industry. It responds to a new energy law that requires new cars and trucks, taken as a collective average, to meet 35 mpg by 2020. “This proposal is going to help us all breathe a little easier by reducing carbon dioxide emissions from tailpipes, cutting fuel consumption and making driving a little more affordable,” Peters said.

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There was a time, when gasoline was cheap, when motorists would often upgrade to premium fuel, hoping to “baby” their cars and trucks. In most cases, it was a waste of money. Vehicles designed to run on regular get little to no benefit from running on higher octane.

These days, however, many motorists are wondering whether they can go the other way, running cars designed for premium on regular, or at least somewhat cheaper regular fuel.

The problem is that there’s been a sudden surge in products running on high-octane gasoline, and not just the traditional European performance cars, like the 2009 Mercedes-Benz SL63 AMG. A story in USA Today notes that 282 ’08 models require high test fuel, up from just 166 in the 2002 model-year. The switch is part of a trade-off, in many cases. Smaller engines generally deliver better mileage, but to maintain an acceptable level of performance, engineers switch to premium gas.

Doing the math may make sense: if you can downsize your engine and bump the numbers from, say, 18 mpg to 22 mpg, you’re getting 22 percent more mileage. But you’ll still feel the pain filling up at a national average of around $3.70 a gallon for high test, versus $3.37 for regular.

What to do? Industry experts recommend you check your owners manual to see what fuel is required, or at least recommended. With many BMW models, you can downgrade your purchase to regular, if you are willing to accept a noticeable drop in performance. But the paper notes that if you don’t use premium in the Mazda RX-8 and Mazaspeed 3 models, you could void your warranty.

Some manufacturers are trying to buck the high-test trend. Cadillac, for example, recently introduced a new, Direct Injection V-6 engine, on models like the 2008 STS and 2008 CTS, that can run on regular without losing power.

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