Maximum cash for clunkers

Vehicles like these will help SUV and truck owners get the biggest bang for their trade-ins.

The reason this rule won't help car buyers is that most cars, even older ones, aren't inefficient enough to qualify as "clunkers." The ones that do, meaning they get 18 mpg or less in combined city and highway driving, are mostly large luxury models that are probably worth more than the $4,500 credit for scrapping them.

The good news is that there have been big fuel economy gains in the SUV market. Even if SUV buyers just buy more SUVs, lots of gas will be saved.

Michael Robinet, head of vehicle forecasting for industry consultants CSM Worldwide, thinks the rules should be even less restrictive to allow more Americans to partake.

"There needs to be a cut-off before a certain model year," said Robinet.

Anything older than that cut-off should qualify regardless of fuel economy. Over the course of a decade or more, Robinet pointed out, cars have improved enough - and old ones have decayed enough - that there are environmental benefits even if the fuel economy's no better. And the economy will benefit from more new car sales.