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Tire Sizes explained:  

Lets look at the following example: P215/70R15

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. In the above example the width is 215mm or 215/25.4 = 8.46 inches. The P is used to show that the numbers are metric.

The second number is a ratio of sidewall height to width. In the above example the height of one sidewall is 215x0.70 = 150.5mm or 8.46x0.70 = 5.92 inches.

The last number is the diameter of the wheel in inches. If it is preceded by the letter R, the tire is a radial tire. In the above example, R15 means a radial tire that fits a 15 inch wheel.

Another version of a tire size might look like G78 X 15. The G stands for the cross-section width, the 78 means that the tire's height is 78 percent of its width, and the 15 stands for a tire that fits on a 15 inch wheel.

If a different size is desired, the overall height or standing height must be kept the same if the speedometer is to read correctly. To find the standing height, multiply the sidewall height by 2 (there are two sidewalls, top and bottom) and add the size of the wheel. An example is 215mm x 0.70=150.5mm x 2=301mm + 381mm(15")=682mm or 5.92"x2+15" = 26.84 inches.
If the standing height is different and you are wondering how the speedometer will be affected, here is a formula:
(New Height / Original Height) X Indicated Speed = Actual Speed with new tires.

Let's look at the original example. If the size is changed from 215/70R15 to 255/60R15 the speedometer will be affected by the following amount:
(27.05" / 26.84") X 65mph = 65.5mph

Therefore this is an okay change if the car is not new enough to have a computer that will be affected by the slightest change in size. A bad change in tire size is switching from a 215/70R15 to a 255/50R15. This would reduce the standing height and make the speedometer read too fast.
(25.04" / 26.84") X 65mph = 60.6mph

Speed Ratings
Q = 99 MPH
S = 112 MPH
T = 118 MPH
U = 124 MPH
H = 130 MPH
V = 149 MPH
W = 168 MPH
Y = 186 MPH
Z = 149 MPH and over 

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This page was last edited on 26 July 2007.

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