Selecting Tires And Rims For Your Mopar - Rubber The Right Way

One of the most important considerations when ordering a wheel for your car is the backspacing of the wheel. The backspacing is the distance between the wheel's flange where it bolts onto the axle, drum, or rotor, and the innermost rim lip where the bead of the tire seats. Measuring the car to see what backspacing is ideal is actually easier than you may think and only requires a straight edge (metal ruler) and a tape measure. At the rear, the car is jacked up so the rearend housing can be supported on a jackstand, which places the rearend in the position it would be in when the car is resting on its wheels. The metal ruler is then placed on the axle flange where the wheel would rest. The tape measure is now used to determine how much offset, or backspacing, is needed to center the tire in the wheel opening. Be sure to measure for backspacing at the maximum diameter of the tire you'll be using and take into consideration both the inner and outer wheelwell dimensions, as well as suspension components such as leaf springs. We use the section width or measured maximum width of the tire as a reference for this measurement. Also carefully consider what will happen as the car's suspension is exercised; you'll want plenty of clearance when the suspension travels to its limits as well.

Measuring the front of the car for proper wheel width and backspacing requires a little more work than the rear. Since the front wheels turn side to side, clearance issues can occur if the measurements are not taken properly. We again start by placing a straight edge on the flange of the rotor or drum, and then we use our tape measure to determine the proper backspacing, tread or section width, and rim width that will work in our application. This can be a time consuming task as multiple measurements need to be taken with the steering turned fully in each direction. Again, the car's weight should be on the suspension so the measurements are more accurate. Also be sure to take suspension travel into consideration when measuring for tires. Just because a tire will fit when the car is sitting still doesn't mean it won't rub when the car is loaded and going over bumps, so be conservative and take the time to measure accurately. When rim width and backspacing are determined, be sure to take the suspension components, especially the upper control arm, into consideration. The backspacing of a Mopar's front wheels is usually limited by the upper control arm or disc brake caliper unless very large diameter wheels are used.

Tire Height, Width, and Aspect Ratio
With all the measuring completed and our wheels properly chosen for diameter, width, and backspacing, we can order our tires, right? Well, we're not ready to order yet. All of our measuring for tire and rim fitment has been in inches, the standard form of measurement in the United States. If you've measured everything like we did, you probably have a diameter and width in inches of the tire you'd like to have. Unfortunately, most tire companies list their tires in the modern "P-metric" format and don't give the tire's dimensions in inches. Luckily, there is a formula that will derive the tire height in inches from the P-metric coding on the side of the tire, but first let's explain what the P-metric coding numbers actually mean. As an example, we'll use a common upgraded rear tire for B-body Mopars, the 275/60R15. The first number, 275, is the section width in millimeters of the tire. The next number, 60, is the aspect ratio of the tire or the ratio between the height and section width of the tire. In other words, the sidewall of the tire will be 60 percent of the tire's width, giving this tire a shorter, or lower, profile sidewall than the same width tire with a 70- or 75-aspect ratio. But how do we convert these numbers to inches? Knowing how a tire's dimensions are defined is the first step.