What do you do to improve the world's best-selling car? And should you bother doing anything at all? After all, why mess with success.

This is the position Ford Motor Co. finds itself with the Escort, which, in addition to being the best-selling car in this country for the decade, has been the number-one nameplate in the world for six years running.

According to the latest figures, more than 6.5 million Escorts have been sold worldwide since the car was introduced in 1980. In the United States alone, more than 2,650,000 were sold during this period. Escort has been sold in more than 60 countries. It is built at assembly plants in Wayne, Mich.; Edison, N.J.; Saarlouis, West Germany; Valencia, Spain; Halewood, England, and Sao Bernardo, Brazil. About one of four Ford cars built worldwide is an Escort. Three of every 100 cars bought worldwide are Escorts.

So, now you know why you see Escorts all over the place.

At a fast glance all Escorts may look alike. But over the years there have been refinements, subtle styling changes and a general upgrading of the product. The car still represents a good buy for the money, which, of course, is one of the reasons for its appeal.

The test car is an LX 2-door hatchback 1988 1/2 model. The mid-year model went on sale in July and is essentially the same as the 1989 model. Design changes for the '88 1/2 include new plastic bumpers, new fenders, new bodyside molding design and a new taillight treatment. Standard 14-inch wheels and low-rolling-resistance tires replace the previous 13-inch wheels on Pony and LX models. Other functional improvements include a new electronically tuned AM radio in place of the mechanical unit and alert chimes replacing a buzzer. The interior has also been freshened with new seat trim and upholstery. Corrosion protection also has been improved through expanded use of galvanized steel and epoxy adhesives.

The only changes for the 1989 model will be gas-pressurized shock absorbers and an engine malfunction-warning light in place of the oil-warning light.

Basic dimensions include a wheelbase of 94.2 inches, overall length of 169.2 inches, width of 65.9 inches, height of 53.7 inches and curb weight of 2,258 pounds. With an EPA index of 102.4 cubic feet, the Escort is rated a compact (100-109 cubic feet). Cargo area measures a generous 16.4 cubic feet with the rear seat in place and an impressive 37.5 cubic feet with the rear seat folded.

The Escort is rated as a four-passenger vehicle and all four will have room. Head room and leg room are exceptionally good for a car this size. Head room measures 38.1 inches in front and 37.3 inches in back while leg room is listed at 41.5 inches, maximum, in front and 33.9 inches, minimum, for rear seat passengers. So, as one can see, if the front seats aren't positioned all the way to the back, rear seat passengers won't have to worry about drawing in their legs.

Although basically an economy car, the Escort is dressed up decently. Upholstery, instrumentation and trim have sort of an upscale look. Nothing really fancy but certainly not bad for the money.

The front-wheel-drive Escort is an easy-to-drive vehicle and even more so when equipped with an automatic transmission as was the test car. It is not difficult to figure out why this car is so popular with first-time buyers and young drivers.

Handling is smooth and responsive. The four-wheel independent suspension features MacPherson struts up front with strut-mounted coil spring, forged lower control arm and cast steering knuckle, while the rear has a modified MacPherson strut and coil spring mounted on stamped lower control arm. Steering is by a rack-and-pinion system which will go from lock-to-lock in 3.04 turns. Rounding out everything are P175/70RX14 radials mounted on 14 by 5-inch wheels.

This setup should please most drivers. But ifo e wants to make things a little livelier, there's always the GT model, a sportier version that features a handling suspension package and a more powerful engine.

And speaking of engines, the Escort is powered by a 1.9-liter/115-cubic- inch , fuel-injected four-cylinder rated at 90 horsepower at 4,600 rpm and 106 foot pounds torque at 3,400 rpm. Now 90 horses doesn't sound as if it will be a big competitor in the street wars but most drivers will probably be surprised on just how peppy the car is. This, of course, wasn't done with mirrors but rather with a final drive geared on the low side - 3.73 for the automatic transmission. Just tromp on it and it doesn't take long before those front wheels chirp.

And, with or without the chirping, fuel mileage is quite decent. The test car averaged 20 miles per gallon for city driving and 31 mpg over the highway. All on unleaded regular.

(The GT's 1.9 HO engine is rated at 110 horsepower at 5,400 rpm and 115 foot pounds torque at 4,200 rpm.)

Base price for the LX two-door hatchback is $7,127, not a really large sum of money these days. Final price on the test car including a delivery charge of $309 and special added discounts of $713, came to $9,593. This was for a nicely equipped car.

Options included Special Value Package 321A (automatic transmission, wide vinyl bodyside moldings, AM/FM stereo with four speakers, digital clock with overhead console, tinted glass, interval windshield wipers, rear window defroster, instrumentation group, light/security group, dual electric mirrors and luxury wheel covers), $1,651; rear window wiper/washer, $126; tilt steering wheel, $124; split fold rear seat, $49; speed control, $182; air conditioner, $688, and polycast wheels, $91.

The Escort is covered by a 6-year/60,000-mile warranty on major components and a 6-year/100,000-mile corrosion perforation warranty.