WITH TIME running out, and with little hope of getting big legislative projects through Congress, President Clinton is trying to build an environmental legacy. He has already taken steps to reduce water pollution and to protect federal lands from commercial development. Yesterday he issued ambitious new rules on air pollution. In so doing he took on an adversary that some list among the great public menaces of our times: the sport-utility vehicle.

There is no doubt that cleaner air makes for better health; even the auto makers and oil refiners, who will bear the brunt of the new regulations, freely concede that. The question, as with all regulations, is whether the benefits are big enough to justify the costs. The answer appears to be yes, though cost-benefit analysis is never an exact science.

The administration proposes to cleanse air in two ways. First, it demands that better catalytic converters be fitted to the tailpipes of cars and -- more notably -- to SUVs, light trucks and mini-vans, which together account for fully half of new vehicle sales. Both the administration and the car makers say this will raise the cost of the average car by less than $100; trucks and SUVs, which have hitherto been subject to lax standards, will cost between $200 and $350 extra. Second, the administration orders oil refiners to reduce the sulphur content of gasoline by about 90 percent. It claims this will boost pump prices by two cents per gallon; the refiners say three to five cents.

The total cost of the new rules may come to between $5 billion and $8 billion a year, depending on whether you believe the administration or the industry. Either way, that is considerably less than the financial benefits that cleaner air promises. The administration puts those at around $25 billion a year, a number that reflects the production of workers who currently die prematurely because of pollution, as well as savings from reduced medical costs and work absences. Even if the actual savings turn out to be just a third of the administration's number, the regulations will have paid for themselves.

Environmental standards often come at the expense of economic growth: Hard trade-offs must be made between poverty reduction and the protection of the planet. But in some instances environmental regulation is a win-win proposition. Cleaner air appears to be one such case, and the Clinton administration is to be commended for pushing ahead with it.