The Geo Prizm is not only the newest nameplate in the automotive world it is also the first of the 1990 model year cars to be introduced.

No, don't check your calendar; you're not behind. The Geo Prizm just happens to be ahead.

For those who have never heard of the Geo Prizm, and right now there are many, it should be pointed out that the car has been around and has been successful. In its previous life it was the Chevrolet Nova. But now, instead of the familiar Chevy bow tie as an emblem, it has its own distinct badge, which is somewhat earthlike in design complete with some longitudes and latitudes.

The Geo Prizm will still be sold through Chevrolet dealers (the test car was supplied by Scott Chevrolet, Emmaus), only now it will be tied in with other Geo models (Metro, Tracker and Spectrum, which are manufactured in Japan, thus the global theme) instead of Chevrolet. But as long as it doesn't cost the consumer any more money, it really isn't a problem.

Prizm is actually different from other Geo models in that it is built in the United States through a joint effort of General Motors Corp. and Toyota. This venture, which has the corporate name of New United Motor Manufacturing Inc. (NUMMI), has its manufacturing plant is in Fremont, Calif. So when you really get down to it, the Prizm, as the Nova before it, is essentially a Toyota Corolla built in America.

The test car was a four-door sedan, but the Prizm is also available as a four-door hatchback. Basic dimensions include a wheelbase of 96.7 inches, length of 170.7 inches, width of 65.2 inches, height of 52.4 inches and curb weight of 2,321 pounds. The car is classed as a subcompact (between 85 and 99 cubic feet index volume), although with an index volume of 95 cubic feet (84 passenger/11 cargo) it is not that far away from a compact.

Front seat room is good and, if the front seats aren't extended fully aft, three passengers will have some leg room in the back. The trunk is what can be expected of a subcompact - not big, not really small. A convenient feature and part of something called the LSi option is the rear seatback, which has a 60/ 40 split and folds down to provide flexibility in cargo carrying.

The interior itself is quite impressive and has an upscale look to it. The contoured front seats have high side bolsters and, like the rear seat, are covered with cloth upholstery. The rear seat has three-point seat belts for the two outer passengers. There are also a number of storage pockets and spaces throughout the interior.

As should be expected in a car this size, driving is a snap; even more so if equipped with an automatic transmission, as was the test car. Put it in drive and away you go. Handling for this front-wheel-drive car is responsive, though not overly so, and parking isn't any big problem. The four-wheel independent suspension - MacPherson struts all around - offers enough to even give the driving enthusiast some fun. The standard tire is a P175/ 70R13 all- season radial.

Styling is in a contemporary aerodynamic fashion and quite easy to take. Rounded lines, aero-style composite halogen headlamps, a sloping hood, a raked, flush-mounted windshield and integrated front and rear bumpers give the Prizm sort of a sporty look.

Interestingly, the unit body chassis is constructed of two types of rust- resistant, high-strength steel, consisting of front/rear subframes and additional reinforcement in 11 critical areas. According to Geo, the result is a highly rigid body that is not only strong and durable, but low on noise and vibration.

The Geo is a lively performer. This can be directly credited to its dual overhead camshaft four-cylinder engine, which is the standard engine for this car. Four-valve-per-cylinder engines have been around for a long time (would you believe the 1910s?) but mostly in limited use. In the n t too distant past, Italian and British manufacturers produced most of these engines.

Now many manufacturers have gotten into the act. And the reasons are quite simple. This type of engine develops high horsepower for its size, is very strong and, perhaps most important, is a ''clean'' engine that adapts very well to emission controls. In recent years Toyota took the lead in the number of these engines offered, so not surprisingly, that's why it is in the GEO.

The engine used in the GEO is not very large; it only measures 1.6 liter/ 98 cubic inches, but it produces 102 horsepower at 5,800 rpm and 101 foot pounds torque at 4,800 rpm. This is a respectable amount of power for a car this size and worked out just fine with the test car's three-speed automatic transmission. Fuel mileage averaged 28 miles per gallon for highway driving and 21 mpg around town.

Base price for the GEO is $9,660 and includes a number of standard features. The test car had a bottom line of $13,111, which included a destination charge of $335.

Options included the Preferred Group #4 Package (air conditioning, power steering, power door locks, AM-FM stereo, cruise control, tilt steering and intermittent wipers), $1,849; LSi interior/exterior trim, $440; automatic transmission, $420, and upgraded radio, $285.

The Prizm is protected by General Motors' 3-year/50,000-mile ''Bumper-to-Bumper Plus'' limited warranty.