Amid the thousands of motorists using suburban Maryland roads yesterday were 142 people in 71 vehicles who pored over coded instructions and manipulated calculators as they tried to follow an intricate 130-mile route to a destination only a dozen miles from their starting point.

At one-minute intervals, starting at 9:30 a.m. the 71 vehicles pulled away from the intersection of Rte. 198 and Rte. 29 in Burtonsville.

About five hours later, near Snowden River Parkway in Columbia, the first of the vehicles completed its run through the summer heat and over the back roads of three counties.

It was a competition, but not a race. Victory was not to the swift, although neither did the laurels go to the the slow. The event was a rally, the fourth and last scheduled for this summer by the Montgomery County Recreation Department and a motoring club known as Branded Inc.

The idea of an automotive rally is to follow the instructions supplied, with the navigator in each car converting printed letters and numbers into miles and turns that will enable the driver to bring the car over the required route.

Completion of the course is necessary, but not sufficient. The driver must come as close as possible to the time allotted for each leg of the run. That is where the computers and calculators come in, enabling the competitors to convert speeds and distances into the minutes and seconds that spell the difference between success and failure in rally competition.

The wilting heat, the unfamiliar course--through Montgomery, Howard and Frederick counties--the cryptic instructions and the constant need to follow them, watch for landmarks and compute elapsed times made the afternoon a challenging and even trying one for many contestants.

For rally veterans Pete and Barbara Stevens, the first real challenge appeared to come amid the cornfields of Plantation Road in Frederick County, about 44 miles into the run.

"Could we have missed something?" Barbara, who was at the wheel of the couple's 1979 Peugeot, asked Pete, who was navigating.

Pete, a systems programmer for the Bureau of Labor Statistics, strongly suspected that they were off course.

"Obviously we missed something," he answered.

"Well?" asked Barbara, who works in member relations for the National Geographic Society.

"Well, turn around," said Pete, who seemed to an observer in the couple's car to betray the first hint of exasperation.

Just after the Stevenses stopped to retake their bearings another vehicle, in which two young women, recognized as fellow contestants, came down the road.

"Do you have any idea where we are supposed to be at all?" one of the women asked Barbara Stevens.

Stevens acknowledged that she did not. The two women, driving a white Pontiac, drove off in the direction opposite to that in which the Stevenses had been headed.

Such an event, suggesting that at least one other highway navigator was obviously lost, "really challenges your confidence," Pete Stevens said later.

Eventually the Stevenses found their error, corrected it and reached the end point. Under the rally scoring system, they were assessed a number of points that indicated they were about eight minutes late--worse than some contestants who finished only seconds from the allotted time, but better than others who had gone astray for 30 or 40 minutes.

"It's fun, it's a structured activity, better than sitting and reading the Sunday paper or going to the pool," said Barbara Stevens.

The winners were David and Breanna Gomberg of Bethesda, who were clocked as finishing 1.8 seconds early.