How do you know what size wheel and tire will fit your car? Here is where a lot of guys get confused about the measurements. You'll need to know how much backspace will fit, the overall width you have room for, and what diameter wheel and bolt pattern of the wheel you need. To measure for the backspace, take a straight edge that is at least as large as the wheel/tire combination you plan to use, and place it against the brake drum (or rotor face if using disc brakes) where the wheel mounts. Now you can take a measuring device (tape measure or ruler), and measure the distance from the straight edge to the part of the car's suspension, brake system, or whatever, that is closest to the straight edge, AND that is outside of the wheel's diameter. (See Image 1.) You'll also have to make sure that if using an aftermarket disc brake kit, that the brakes will fit inside of the rim's diameter.
If you measure from the inside diameter of the rim's bead-edge, there should be nothing within 1 1/4 inches of it. We have found this gives the tire enough room for expansion when mounted, and flex when cornering.
The next measurement you need to get is the overall rim width (see image 1). Again, measure using the straight edge and a ruler to find out how wide a wheel will fit in your opening--remember to leave about 1 1/4 inches of clearance on each rim edge (inner and outer).
When you are comfortable with the measurements you have taken, now you need to decide what tire will fit your wheel and opening. Tires have a couple of measurements you need to pay attention to--section width, tread width, sidewall height, and overall diameter. Section width is the actual width of the sidewall at its widest point after the tire is mounted and inflated. It's this measurement that gives cause for the 1 1/4-inch spacing between the rim edges and the vehicle. Tread width is self explanatory, it's the width of the tread pattern. Sidewall height is the distance from the rim to the tread, and overall diameter is, well, the diameter of the tire after mounted.