Shopping for car tires is more difficult today for many Washington-area consumers because of the suspension of the federal program requiring tread-wear ratings.

Many manufacturers had objected to the requirement that the ratings be stamped on the tires on the grounds that the rating system established by the National Highway Traffic Safety Administration was sometimes inaccurate and often injurious to sales of their product.

In announcing the suspension of the system in February, Raymond A. Peck, then administrator of the agency, said, "There is such variability in tread-wear test results that meaningful grade assignment simply cannot be assured. We just cannot tell whether a higher-graded tire will wear better than a lower-graded tire."

Consumer groups and a few manufacturers--notably, Uniroyal Inc.--disagreed with Peck and fought to keep the tire tread grades, which were assigned by manufacturers based on road tests over specific courses.

Uniroyal frequently used the rating reports in its advertisements, and when the requirement was dropped, Robert H. Horning, marketing vice president of Uniroyal Tire Division of Uniroyal Inc., reacted sharply, saying the consumer "is being shortchanged" by the suspension.

Motorists feel the impact of the NHTSA decision as they puzzle over the many tire claims, sales and promotions and try to judge the performance of one brand compared to another.

The advertisements are especially appealing to families making vacation plans and looking for good tires that will let them avoid the flats and blowouts that can be inconvenient and dangerous in cars loaded with luggage, children and all the other paraphernalia of travel.

But while the tread-wear rating information is no longer required, some helpful tire buying guides are available. Two of them are:

"New Consumer Tire Guide," a 12-page pamphlet published by the Tire Industry Safety Council, a trade group.

For a free copy, send a stamped, self-addressed, business-size envelope to the Tire Council, Box 1801, Washington, D.C. 20013.

"Tips on Tires," a 10-page pamphlet published by the Better Business Bureau.

For a copy, send 25 cents and a stamped, self-addressed, business-size envelope to the Council of Better Business Bureaus, 1515 Wilson Blvd., Arlington, Va. 22209.

In addition, the Tire Information Center, a New York-based industry group, publishes a book, "The 1983 Tire Guide," which lists the mileage you can expect from a particular tire brand.

The American Automobile Association's Potomac Division has a copy of the book in its library and will look up brand performance upon request, said Robert Livingstone, manager of the AAA Approved Auto Repair. The number for Washington-area residents to call for tire mileage information is AAA-CARS.

Depending on the quality of the tire you buy, you can spend $30 to $80. The general rule for tire prices, Livingstone said, is "the more you pay, the more miles you can expect to get."

He suggests that motorists read the tire advertisements and talk to tire dealers to find the best deal. In some cases, that may be an inexpensive brand. "If you have an older car that you use only around town, you can get by with a $40 radial," he said.

However, anyone who does a lot of highway driving and who plans to keep his present car may be better off with a heavier, more expensive radial, he said.

In either case, proper tire inflation is critical to the comfort of the ride, the life of the tire, the gasoline mileage and even car safety.

But underinflation, which is far more common than overinflation, can result in excessive heat, seriously reducing tire life and causing tire failure. Moreover, underinflation can result in the car's burning more gas than usual.

"You can lose two to three miles a gallon on your gasoline mileage with tires that are underinflated," Livingstone said.

Yet, because of the declining number of full-service gasoline stations, many motorists make the mistake of running their cars with underinflated tires. Livingstone said about one-third of the cars checked by the AAA Diagnostic Service have underinflated tires.