New car fever is attacking consumers everywhere this month as Detroit rushes its latest offerings of cars to dealer showrooms. The main introduction is scheduled for Sept. 29, although a few 1984 models already have been unwrapped and put on public view.

But Jack Gillis, the Washington author of "The Car Book," a widely acclaimed consumer digest of car facts, said consumers shopping for the best buy should look first at the 1983 models.

"This is the best time to buy a 1983 model because dealers are trying to make space for 1984 cars," he said.

Foreign cars, particularly those made by the Japanese, are in short supply and may be hard to find, Gillis said, and some larger American models also may be scarce because of a recent surge in sales. But, in general, he added, "There are a lot of '83 American domestic models available."

Other tips from Gillis:

* Resist the temptation to buy immediately any new model that is in its first year of production. "It takes six months to a year for manufacturers to work out the bugs in a new model," he said, so consumers are better off waiting at least six months to buy such cars. "We know that because the most problems with safety and defects generally occur during the first year of production," he said.

* Remember that many 1984 cars will have weaker bumpers because of the elimination of the federal requirement that they be able to withstand crashes of up to 5 mph without damage. "Ford is the only manufacturer equipping its entire line of automobiles with the bumper that can withstand the 5 mph crash," he said. "The other manufacturers have weakened the bumpers on at least some of their new model cars. And Volvo and Honda have eliminated the 5-mph bumper entirely from their new cars."

* Expect better quality from the 1984 American cars. "There is a distinct improvement over what Detroit has produced in the last two or three years," Gillis said. However, he said the U.S. models still aren't up to the quality of Japanese cars.

Gillis is the marketing analyst who helped write the first "Car Book" in 1980 under the auspices of the National Highway Traffic Safety Administration, which then was being adminstered by consumer activist Joan Claybrook. That book, which was published by the Carter administration and was distributed free to consumers, was designed to help shoppers make the best choice when buying a car. Filled with information on car safety and performance records, maintenance expenses and insurance costs, the book was an instant hit.

But due to budget reductions by the Reagan administration, the book was not updated.

Gillis refused to let the book die, however. Convinced that the "Car Book" provided information that consumers couldn't easily get elsewhere, Gillis left his government job and wrote the 1982 "Car Book." The next year he wrote the 1983 "Car Book."

Both included analyses of new cars, listing the results of crash tests for individual models, their fuel economy and operating costs. The expanded 1983 book contains special sections on resolving complaints, shopping for a used car and buying auto insurance.

At the end, Gillis outlines buying tips, including:

* Don't trade in your old car. You can almost always do better by selling your car yourself than by trading it in.

* Buy from stock on hand. You generally get a better deal by buying a car the dealer has on the lot. But if it has expensive or flashy options that you don't want or need, ask that they be removed.

Avoid delicate options that often have a high frequency-of-repair record, such as power seats, power windows, power antennas and, especially, special roofs.

* Don't rely on the sticker price. While the exact figures vary, the difference between dealer cost and sticker price is 15 to 20 percent. For foreign cars, unfortunately, because of their popularity you may have to pay more than the sticker price.

* Examine the car on delivery. A few minutes of careful inspection can save hours of aggravation later. Carefully look over the body for any damage, check for the spare tire and jack equipment, make sure all the electrical items work and make sure that hubcaps and body moldings are in place. You may want to take a short test drive. And, finally, make sure you have the owner's manual, warranty forms and all legal documents.

A copy of the 1983 "Car Book" is available for $8.95 from the Center for Auto Safety, 1223 Dupont Circle Building, Washington, D.C. 20036. The 1984 edition of the book is being compiled and is scheduled to be available for distribution in January.