The Senate’s “Cash for Clunkers” bill passed Thursday, as an addenda to the war-spending bill. It was approved by the House last week. The bill allows for trade-in vouchers of up to $4,500 for owners of gas guzzlers who want to buy or lease a new, more fuel-efficient vehicle.   If signed into law by President Obama, the program is likely to go into effect in August.

Called “Handouts for Hummers” by critics, the program is less green than advocates had hoped, and will not do an extraordinary amount to limit the United States’ dependence on foreign oil. Car owners could get a $3,500 voucher for trading in an under-18 m.p.g car for one that gets at least 22 m.p.g., or $4,500 if the new car gets 10 m.p.g. higher than their existing vehicle. Truck, minivan, and SUV owners will receive a $3,500 voucher for buying a similar new vehicle that gets at least 2 m.p.g. higher; that voucher will increase to $4,500 if the new truck/van/SUV gets at least 5 m.p.g. higher than their existing vehicle.

There are some concerns that the program is underfunded: the bill provides $1 billion for July through November of 2009. Keith Johnson of the Wall Street Journal posits that the program could cost about $4 billion.

Not only that, but even GM’s CEO, Fritz Henderson, said the legislation would help increase new-car sales by only 10 percent, compared with Germany’s 40-percent rise in new-car buying after a similar program was introduced in that country.

Finally, if your car has a trade-in value that is higher than the $3,500-$4,500 voucher amount, you’ll probably be better off forfeiting the voucher, although some cash-starved car dealers might think of ways to maximize your trade-in.