STRAIGHT TALK COLUMN: Size matters when replacing tires

Straight Talk: Bruce from Florida asks: “I own an old (1987) Ford Mustang convertible that I love and it needs tires. I would also like to replace the wheels but I want to keep raised white letter tires that are only available for 14 inch and 15-inch wheels and I have 14-inch wheels now that are original. The tire and wheel shops around me want to sell me larger diameter wheels but then I would not have raised white letter tires. Any help would be appreciated.”

Halderman: Owners of vehicles are often confused when it comes to replacing tires. The things that need to be checked when replacing tires include:

  • The correct size. A tire size describes the width of the tire, the height of the sidewall, and the rim diameter. This should be the same size that was originally on the vehicle.
  • Speed rating. The speed rating is given as a letter (Q, T, S, H, V, W, and Y) and should be the same as the original tires that came with the vehicle.
  • If the vehicle will be driven in a climate that is above 45°F (7°C), summer-type tires can be used. If the vehicle will be driven under a variety of conditions and both above and below 45°F (7°C), all-season type tires should be selected.

If changing to a larger wheel, such as to 15-inch wheels so that you can get the raised white letter tires, it is important that the same outside diameter of the tire/wheel assembly be kept the same. This is important because it can affect the speedometer and the shifting of the automatic transmission as well as headlight aiming.

Most tire and wheel stores will select the proper size wheel/tire combination that will work and keep the same diameter as the original wheel/tire combination.

Most wheels are designed to fit over and be supported by the axle hub. Some wheels use an enlarged center hub section and rely on the wheel studs for support and to keep the wheel centered on the axle. Some aftermarket wheels may be designed to fit several different vehicles.

As a result, the wheel manufacturers use plastic hubcentric adapter rings to adapt the wheel to the vehicle.

A useful website I have used to help narrow down the proper size needed is

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