If fuel economy is a prime consideration in your car purchase, these factors, says Jack Gillis in The Car Book, can affect your mileage:

* Transmission: If used properly, manual transmissions generally are more fuel-efficient than automatics. "A four-speed manual transmission can add up to 6.5 miles per gallon over a three-speed automatic."

* Axle ratio: It's the relationship, says Gillis, between the revolutions of the drive shaft and the revolutions of the wheel. "Generally, a low ratio, such as 2.69, means less engine wear and better fuel economy than a higher ratio, such as 3.47."

* Engine: "The smaller the engine, the better the fuel economy."

* Tires: Radial tires "can improve your mileage from 3 to 7 percent over conventional bias-ply tires."

* Cruise control: Driving at a constant speed "uses less fuel than changing speeds frequently."

* Air conditioners: Because of their weight, they require more horsepower to operate. "They can cost you up to 3 miles per gallon in city driving."

* Trim package: "Upgrading a car's trim, installing soundproofing and adding undercoating," says Gillis, "can increase the weight of a typical car by 150 pounds."

* Power options: Power steering, brakes, seats, windows and roofs also add weight. "Power steering alone can account for a 1 percent drop in fuel economy."