THE 55 MPH speed limit was adopted by Congress seven years ago to save gasoline. It has achieved that, reducing consumption by three or four billion gallons a year. But it has had an even more important side effect. The best estimates are that 45,000 Americans who are alive today would have been killed in highway accidents without it. This law, designed to save energy, has become one of the most important public health measures of the 20th century.

Despite the piles of statistics that support that conclusion, efforts are under way both in Washington and in some state capitols to go back to the days of zipping through the countryside. The Reagan administration wants to turn the matter back to the states. Some state legislatures, particularly in the West, are eager to raise the speed limit once Washington gets out of the way. The Montana House, for instance, voted just last week to set the limit at 70 on interstate highways.

Since the 55 limit was imposed the number of vehicles and the number of miles they are driven have gone to about 30 percent. The number of fatalities has gone down about 7 percent. The chance you will be injured in a traffic accident is about 5 percent less now than it was in the early 1970s. But the chance you will be killed is about 20 percent less. Life insurance actuaries say the sudden increase in life expectancy since the early 1970s is largely attributable to the reduction in highway deaths.

Is that reduction due to the speed limit? In 1973, 55 limit was imposed, 45,192 people were killed. While the number of miles people drove dropped 3 percent between the two years, the number of fatalities dropped 16 percent. And the number of deaths stayed down when driving increased in subsequent years.

Those who want to repeal the federal law or raise the limit most produce some overwhelming reason to disregard that evidence.