Mazda has a hot seller in the all-new 626

At Boston Mazda, business was never so good or back orders so many, except maybe for the rotary RX-7. Bob Rudin, sales manager for the nine-year-old car dealership, reports a four- to five-month delay in delivering an all-new 626. ''Sales are booming,'' he declares. Last year Boston Mazda delivered 433 new cars, but ''we could have sold another 600 if we could have gotten them,'' sighs Eugene F. Looney, the owner.

And price seems to be no deterrent, adds Mr. Rudin, who reports the front-drive 626 ''is going out the door at about $8,500.'' Well, having driven a 626 for two weeks, I turn over the key only reluctantly. The car is that good.

The new car comes in three versions - a 2-door coupe, 4-door sedan, and 5 -door hatchback. While it is related to the car it replaces because of name, it is really all new, including engine and drive train, chassis, and design. Even the dashboard instrumentation is better than before.

Basic to the design of the new 626 is a beefier engine to meet some of the criticisms of the car it succeeds. Internal engine friction is reduced, according to Mazda engineers.

A new dash-mounted switch allows the driver to choose the shock-absorber damping rate, depending on the type of road over which he will travel. I couldn't feel any difference in straightway driving on the Interstate, however, but the effect shows up if the need arises for a quick maneuver on the road.

The car uses disc brakes in the front and drums in the rear.

Inside space is more generous than before. Multiple seat adjustments in the luxury trim package should make any motorist happy. It may be the closest thing to a Recaro form-fitting seat now on the market.

The new 626 is a stable car, does what it is told, and rates highly in interior roominess, passenger comfort, driver handling, styling, and economy. A higher-performance model could be on the way as the market emphasis on high fuel economy continues to slack off.

For the electronic wizard, a digital dash is an option. As for m.p.g. on the road, I figured between 32 to 34 in a week of daily commuting, plus some weekend miles on the expressway.

No one can fault Toyo Kogyo for confusing the marketplace. The company sells three basic cars in the United States, unlike Toyota and Nissan with their much wider array of cars. If a customer wants a Mazda, his choice is simple. The front-drive GLC econocar parks at the bottom of the line; the 626 coupe, sedan, and hatchback make up the company's ''world-class vehicle''; and the rotary-engine RX-7 its sports car.

The 626, in fact, shows the direction the company will take in product research and development in the future, according to Yoshiki Yamasaki, president of Toyo Kogyo.

''Toyo Kogyo intends to expand its reputation as a manufacturer of distinctive products which incorporate the latest automotive technology while providing customers the best possible value,'' he asserts.

''We believe that the new, international midsize Mazda 626, which features aerodynamic styling and excellent handling and comfort, will be seen as the next step in Mazda's tradition of providing customers with unique, technically sophisticated, high-quality products.''

The 626 is built in the company's new production plant in Hofu City, some 56 miles from the company's headquarters in Hiroshima, Japan. It's the first time since the company was founded 51 years ago that Mazda vehicles have been built in a location other than Hiroshima.

The company built 1,202,583 units in 1982, including more than 84,000 ''knock-down sets'' which are shipped to underdeveloped parts of the world for assembly.

Meanwhile, the front-drive Mazda GLC topped the 1 million mark last July after only 27 months in production, a first for any Mazda model.