The big automakers have been speeding down a straightaway for the past year, enjoying fat times reminiscent of the preimport days. Tagging along behind the carmakers, with feet pressed to the gas pedal, are nearly 2,500 small makers of car replacement parts.

The replacement business should sparkle as all the new metal on the road starts to dent, rust, and rattle. New car sales climbed 25% last year to more than 10 million units. "The real boom for us doesn't hit until three years after the model year," says Stephen Barbone, spokesman for the Motor and Equipment Manufacturers Association.

Business isn't bad right now for makers of repair parts. The average age of cars on the road is seven years, up from five years a decade ago, and that means demand is increasing for replacement parts. Until last year, when the economy brightened, consumers put off making discretionary repairs. "Mostly what we saw then was a flattening of business," says Robert Hirsch, president of Gold Eagle Co., a Chicago manufacturer of such automotive chemicals as fuel additives. "People were keeping their cars longer, but they couldn't afford to take care of them."

Some auto experts, though, warn that a cloud hangs over the potential boom in replacement parts. Joseph Phillippi, an analyst for E.F. Hutton & Co., notes that the total number of cars in the 3-to-11-year age bracket peaked in 1982 and will gradually decline through 1987. Phillippi predicts that there will be a "mad scramble for market share" in the future. Even so, Hirsch and his competitors are building up inventory and preparing for a move into fourth gear.