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Have you ever wondered what are all those numbers on the sidewall of your tires? Let us help you decode those numbers.

Picture Credit: F l a n k e r

Tire Sizes Explained

Width of tire (Example: P 215/65R15 95H)

The three digits following the service type prefix (if present) tell us the cross-sectional width of the tire in millimeters.
The width of the tires, measured from the widest point of the inner sidewall to the widest point of the outer sidewall, is 215 millimeters.
The section width can be converted to inches by dividing millimeters by 25.4: (215 millimeters) / (25.4 mm/in) = 8.46 inches.
Aspect ratio (Example: P 215/65R15 95H)
Next, we have the two-digit number that usually follows the tire’s section width tells us the aspect ratio, or tire profile measurement.
The 65 indicates that the height of the sidewall from the rim to the outside of the tread, is 65% of the section width.
We know the tire size above has a section width of 8.46 inches and the aspect ratio is 65%, therefore the sidewall height for this tire is 5.5 inches: (8.86 inches) x (.65) = 5.5 inches.

Diameter of Tire and wheel (Example: P215/65R15 95H)

The next number we are going to look at beside is R15. The number 15 tells us that the tire should be fitted on to a 15 inch diameter rim.
The most common tires sizes are (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28.

Special wheel diameters

Other than the common sizes you usually encounter, there are also some special tire sizes out there.
We can usually find these tires on some heavy-duty light trucks, box vans, and heavy-duty trailers.
These sizes are usually 14.5, 15.5, 16.5, 17.5, and 19.5 inches, and an example would be 33×12.5R16.5 118R.
We should always check the tire and rim size before mounting them together and never mix special size tires with traditional size rims.
Similarly, we should also not mount traditional tire size on special size rims.

Service description rating (Example: P215/65R15 95H)

The next two digits we will be looking at is what we call the service description rating. Since 1991, It is mandatory to print it at the end of the tire size with the exception of Z-rated tire.
Thus, looking at the above example, Service description is used to identify the tire’s load index (95) and speed rating (H).
We will cover the load index and speed ratings on our next blog. Stay Tune!
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