Tire buying used to be so simple! A generation or so ago there were two kinds of tires for your car-one with black sidewalls and the other with white sidewalls.

Today, the motorist has a choice of as many as a dozen different types of tires in a given size.

And, often the motorist is not quite certain what tire would be best for his car and his type of driving.

Engineers of The Firestone Tire & Rubber Company were asked a series of questions and have answered them in an effort to help tire buyers learn more about their tires and tire construction.

Q. What's a tire made of?

A. An average sized passenger tire weighs approximately 23½ pounds. In it are 11 pounds of mixed synthetic and natural rubber; almost a pound of steel in the bead wire; 2½ pounds of cord material' 6 pounds of carbon black; 2½ pounds of petroleum oils, and the remainder of the weight is in sulphur and chemical accelerators and anti-oxidants.

Q. How are tires made?

A. On the tire building machine is a drum approximately two feet long with a diameter about the size of a car wheel. As the drum rotates, the inner liner is wrapped around the drum. Then, the tire builder begins applying fabric, ply by ply. When the desired number of piles have been added, the tread and sidewall rubber and the beads are applied. Then, the "green" tire is removed from the drum, and cured in a mold with the tread and sidewall patterns which transform the "green" tire into its final shape and appearance.

Q. What is fabric?

A. It is woven fibers impregnated and coated with rubber. Today's passenger tires employ various man-made fibers, including rayon, nylon, polyester and fiberglass.

Q. Why have more than one type of fiber?

A. The different fibers are used in making tires for various uses. Rayon provides a soft ride without flatspotting, while nylon rates best for those who do a lot of sustained high-speed driving. Polyester combines some of the best features of both. Fiberglass is the strongest of the four, but does not withstand constant flexing. Therefore, it is used only for tread belts, not in sidewalls.

Q. What are the different types of tire construction?

A. Bias-ply, belted-bias, radial and the LXX New Concept.

Q. What is a tire of bias-ply construction?

A. This tire is fabricated from a layer or ply of cords which run at an angle from one rim-edge to the opposite rim-edge. The angle provides the strength. This strength must be balanced symmetrically across a central vertical plane so another ply is applied at an opposing angle. There you have a two ply tire. Add two other layers and it's a four-ply tire.

Q. What's a belted-bias tire?

A. It is built the same way as a conventional bias-ply tire but in addition it has two belts of a more rigid material encircling the tire under the tread. These tread plies make the tire more resistant to punctures and impacts and provide greater mileage.

Q. What's a radial tire?

A. The body of a radial tire is composed of two plies which run straight up and over in "hoop" fashion from rim-edge to rim-edge. It also has two to four belts around the circumference under the tread. The radial tire offers high mileage but with a slightly "stiffer" ride at slow speeds.

Q. What's the LXX?

A. This is an entirely new concept developed by Firestone as the tire of the1970's. It is scheduled to reach the marketplace this summer. The LXX is similar to the belted bias tire in many respects and drastically in others. While it has plies and two belts beneath the tread, it also has belt plies in the lower sidewall. They are cantilevered nearly horizontal from the beads for added stress strength. The LXX has a "deep dish" appearance because of its narrow-width rim, only 3½ inches. Wheels are 16, 17 and 18 inches in diameter, yet the overall diameter of the tire and wheel combination remains approximately the same as conventional tire-wheel combinations.

Q. Different tires have different size markings. Can you explain the markings?

A. Size designations are determined by the Tire & Rim Association. A popular-sized conventional biasply tire is size 8.25-14. The 8.25 indicates the width of the tire section is approximately 8.25 inches. The "14" is the rim diameter. This tire is interchangeable with the newer tire, size G78-14. The "G" in this marking indicates the tire's load carrying capacity at a given inflation pressure; the "78" means the section height is approximately 78 per cent as great as the width, and the "14" is the rim diameter. Either of these tires is interchangeable with the radial marked 205R14. The "205" indicates the tire section is approximately 205 millimeters wide; the "R" means the tire is of radial ply construction, and the 14 is the rim diameter. The LXX has still different size designations. The first three tires are interchangeable with theH50C-17. In the LXX the "H" is the load carrying capacity at a given inflation pressure; the "50" indicates the section height is approximately 50 per cent as great as the width; the "C" means the sidewall is cantilevered, and the 17 is the rim diameter. (Conventional wheels would have to be replaced with LXX wheels to accommodate LXX tires.)