Restoration Tip – How to read tire sizes
Tire size and stance can drastically change the look of your muscle car or classic. When trying to decide on a new set of rubber that is different from the stock size, knowing how to read the the measurements of a tire size to ensure the best fit is helpful in determining what will fit. When you understand these measurements it helps you understand what is changing when you hear all the different number associated with the measurements. These few tips will cover just the basics of reading your tire sizes. I will break down the three basic number that you will encounter on the side wall of you tire.
For our sample tire I’m going to use a Cooper Cobra Radial G/T with a size of 215/70 R15
The first measurement we are going to read is the tires width known as its “section width”. Now remember that this measurement is in Millimeters. This tire is a 215. 215 is the measurement at the widest part of the tire from the outer most edge of one side to the inner most edge of the other side. Since there is 25.4 millimeter in an inch, it measures 8.64 inches at the widest section (215 ÷ 25.4 = 8.464)
Now this is not the tires actual “tread width”. Differing treads styles cause a variation in tread measurements. Each manufacture tread widths can vary from style to style so this does not accurately represent the “tread width
The second number in line is the “Sidewall Aspect Ratio”. This number represents the tires sidewall height. On our example tire, it reads as a 70 series. The number 70 is the percentage of the “section width”. This means the side wall height from the wheel rim to tread is 70% of 215 (the section width). So to convert this to inches we will take 215 x .70 = 150.5mm and then 150.5 ÷ 25.4 = 5.925. 5.925 is the height of the side wall in inches from the wheel rim to the tread. These very simple calculations will help you determine how much changing the side wall of the tire will affect the look and stance of your car.
The final number in this tire size is the “rim diameter”. The rim diameter is the tallest part of the wheel. Other than just matching this number to the actual wheel size this number will help determine overall tire height as well. In order to figure the tire height you will need to have previously figured the sidewall height. The actual tire height will be the sidewall height multiplied by 2 plus the rim diameter which is 15 in this case.
The equation looks like this: 5.925 x 2 = 11.85 + 15 = 26.85. 26.85 is the overall tire height in inches.
Knowing these numbers is a useful tool when trying to contrast and compare an upgraded tire for your restoration, hot rod, or even daily driver. These are just the basics of what the actual numbers mean. There are many other indicators and markings to decipher on the sidewall of a tire, but knowing the basics is one step in the right direction.