You don't have to be a classic car collector to drive off in a '57 Chevy convertible, according to an article in the current issue of Popular Mechanics, you can rent your time machine from National Car Rental's California Classics program.

Although there are old car rental operations of a local nature here and there around the country, the size and scope of the National plan eclipses anything previously attempted.The plan was designed to stimulate membership in the rental firm's Emerald Club, a frequent renter program similar in concept to the airline bonus plans.

The original California Classics fleet began a year ago at National's Los Angeles International Airport facility.

The fleet started out as a mixture of coupes and convertibles from the golden era of American automobiles - the '50s and '60s - but it has since been purified to an all-convertible collection.

It includes a number of two-seat Thunderbirds (1954-57), late '50s Corvette roadsters, several Chevy Bel Airs, a rare `54 Buick Wildcat and three '59 Cadillacs.

Last December National established a fleet of British roadsters at its San Francisco airport headquarters.

It includes some cars that are rare by any standards. Foremost among them is a 1937 MG-TA roadster, forerunner of the MGs that more or less launched the sports car era in America.

Cars are handed out on a first-come-first-served basis.

A note of caution - take a good look at what you are renting before you leave the lot. Many of the cars have developed one or more of the idosyncrasies one associates with old age in automobiles.

In San Francisco, where almost all the cars are right-hand drive, the cars suffer from the time-honored complaints against British sports cars - cramped driving positions, rubbery steering and a tendency to overheat.

One car the magazine rented - a 1937 MG-TA - developed a fuel feed problem and quit running in the

exact middle of the Golden Gate Bridge.

Cars rented in Los Angeles had small deficiencies - a power window that wouldn't roll down, a balky throttle response or a radio that wouldn't work.

National makes no bones about the fact that these are old cars and nothing produces automotive infirmities so effectively as age. A quick checkover will tell you if the car's basic systems are working.

Renting one of these dream machines costs $49.95 a day ($59.95 in San Francisco), plus 30 cents a mile, fuel, extra insurance (optional) and the $50 Emerald Club initiation fee.

Public response to the Los Angeles and San Francisco facilities has been so positive that National has added oldies operations at its outlets in San Diego, Las Vegas and Palm Springs, Calif.

Plans are under way to set up several fleets in Florida. Target cities are Fort Lauderdale, Fort Meyers, Tampa, West Palm Beach, Orlando and Miami.