CONGRESS IS SHOWING just about the right mixture of rigor and flexibility in forcing the transformation of the American automobile. The nation's favorite machine has arrive at the midpoint of what is now a 15-year process of change required by law. It began in 1970 when the Clean Air Act amendments set up a timetable for reducing automobile-exhaust pollution by 90 per cent. That great achievement is now nearing completion. Most of the mandated gains in pollution control have already been made. When Congress finally enacted this year's revision of the Clean Air Act last week, it provided a decent and useful compromise that gives the manufacturers a little more time to reach the final standards. But it conceded very little in the stringency of those standards.

These past few weeks may well have seen the last of the major political struggles over automobile emissions. In the second half of this 15-year schedule, it will be the requirements for fuel efficiency that force the greatest changes. The schedule of efficiency standards that Congress enacted in 1975 comes into effect with the 1978 models that are just about to go on sale. Each company's 1978 cars will have to average at least 18 miles to the gallon; by 1985, they will have to average 27.5 miles to the gallon.

These tightening fuel requirements are not likely to generate the same kind of incessant battling in Congress, or the demands for delays, that the pollution rules did. The reason is, simply, that fuel economy seems to sell cars. You have not seen many ads in which companies claimed that their cars polluted the air less than the competition. But a great many ads now carry miles-per-gallon ratings.

Congress, nevertheless, intends to reinforce the fuel-efficiency rules to prevent future backsliding. The House has just passed the energy bill with a stiff tax on inefficient cars. The Senate Energy Committee went a step further last week by voting to prohibit the manufacture of inefficient cars altogether. The Senate committee is talking in these derogatory terms about cars that get less than 16 miles to a gallon - that is to say, the same cars that people used to describe admiringly as luxurious, heavy, powerful. There's an interesting turn on social history contained in that change of adjectives.

The practical effect of the Senate committee's ban would evidently be small, even though the definition of prohibited cars would tighten from year to year. The companies say that even the existing law requires them to stay above those limits. But the senators fear a relapse into luxury, and they are drawing the regulatory net a little tighter.

Incidentally, the automotive engineers deserve a large share of the credit for the progress now being achieved. The sharpest weapon against the pollution standards has always been the charge that it would degrade fuel efficiency. But rapid technological development has made it possible to cut to zero the price, in fuel losses, for cleaner automobile exhausts.

There's a striking contrast between Congress's firm resolve on automobiles and it paralysis on gasoline. Even as the House was passing the clean-air and auto-mileage requirements last week, it once again voted down - by a huge margin - the addition of a few pennies to the gasoline tax. There's a danger in this disparate treatment. By requiring increasingly efficient cars, without substantially raising the price of gasoline, Congress will succeed only in making it cheaper to drive. As it gets cheaper, Americans are likely to respond by driving more. This summer's soaring gasoline sales make you wonder if that isn't already beginning to happen.