The Car Allowance Rebate System (acronym: CARS) is going into effect today. Commonly known as "Cash for Clunkers," the program gives cash vouchers to people who trade in inefficient cars for new, cleaner ones. The New York Times' Wheels blog summarizes:Essentially, consumers can get up to $4,500 toward the purchase or lease of a new fuel-efficient car or truck if they bring in an old vehicle that gets a combined average mileage of 18 miles a gallon or less. Back in February, Ben Jervey outlined the benefits of such a program:On paper, "Cash for Clunkers" hits something of an immediate domestic policy trifecta: it reduces our dependence on foreign oil; it saves Americans dollars at the pump, allowing for increased consumer spending (and the economic stimulus that generates); and it should help bolster the sputtering market for Detroit's latest offerings.And that's not to mention the environmental benefits, which could be huge. (Does that make it a quadrafecta? A superfecta?) Gather this-American cars more than 12 years old account for only 25 percent of the miles driven in the United States, yet they produce roughly 75 percent of all automobile-born pollution, including greenhouse gasses and low-level pollutants like ozone and fine particulates that wreak havoc on the lungs of urban dwellers.So now that the specifics of the program have been hammered out, how did we do? Bloomberg has more details:Consumers will get $4,500 vouchers if the new car they are buying gets 10 miles-a-gallon better gas mileage than the model they are trading in. For light trucks, the improvement must be 5 mpg better than the older model, and for large light trucks, 2 mpg. For a $3,500 voucher, the improvement for cars must be 4 mpg or better, for light trucks, 2 mpg, and for large light trucks, 1 mpg. The trade-in vehicle must be no older than a 1984 model and get 18 mpg or less in combined city/highway fuel economy. New passenger cars purchased with the vouchers must get at least 22 mpg in city/highway fuel economy, light trucks must get at least 18 mpg, and large light trucks 15 mpg. Domestic as well as foreign models sold in the U.S. qualify. Unfortunately, you can't use one of these vouchers to buy an efficient used car. But the Sierra Club likes the program. They have a calculator up on their site that will show you the money and carbon you could save depending on your current car, new car, and annual miles driven.Anyone out there planning to take advantage of this Cash for Clunkers program? What are you trading in and what are you getting?