In this installment I’ll discuss rear tire fitment
First think of your tires in terms of a rectangle. Not round as viewed from the side, but height x width as if looking down on the chassis.
All tires will have these two absolute dimensions.
Forget for a minute about P-metric tire sizing, I’ll cover that next time.
How wide is it? And how tall is it?
To get a better understanding of what will fit you will first have to measure your wheel wells. No, I’m not going to do it for you. I don’t have the car here, you do.
You will need a plumb bob ( 24”string and a washer tied to it), a tape measure, a ruler or other straight edge (I like to use a carpenters framing square).
For this example I’m measuring the rear of a '69 Dodge Charger with drum brakes.
Remove the wheel but keep the suspension loaded if possible. You don’t want to measure with the rear axle hanging down 4 inches further than normal.
To find the overall width I’m going to make two measurements and then add them together. These will be useful later when the discussion turns to wheels.
Begin by hanging a plumb bob down from the inside edge of the fender lip at the center of the wheel opening. Measure from the now hanging string to the surface of the drum, or what we will call the wheel mounting surface to get the front spacing. . In this example we get around 6 inches. Try moving your string fore and aft and see how the contour of the body brings the fender lip in closer to the drum.
Consider your tire height and where the sidewall may contact. Be conservative in your estimate and your chances for a perfect fit increase.
Write this measurement down and then proceed to the back spacing measurement.
Using your straightedge lay it across the mounting surface and take a measurement back to the closest obstruction. In this case my measurement is a little over 6 inches at the leaf spring, but it is the inboard wall of the wheel well (about 2 o'clock) that is the closest obstruction at around 5 inches.
Adding the front width and the backspacing width I arrive at 11 inches. This does not account for clearance! It only serves as the maximum width.
Keeping a half inch of clearance at both the rear and the front we can safely say that a 10 inch wide tire will fit here.
Remember that the section width is wider than the actual tread width. Always go by section width.
We ended up using a 10" wide 255/60R15 that looks just right with steel wheels on this NASCAR legend.
That’s all for now. Next installment will focus on tire diameter.