Tire Size Calculator
Calculate tire diameter, sidewall height, and revolutions per mile.
Enter tire size numbers from the sidewall (e.g. 225/65R17 = width 225, aspect 65, rim 17)
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Understanding your tire dimensions helps with fitment decisions, speedometer accuracy checks, and comparing plus-sizing options. The numbers on your tire sidewall tell you everything needed for this calculation.
- Find your tire size. It is molded into the sidewall in the format: width/aspect ratio R rim diameter. For example, 225/65R17 means 225mm wide, 65% aspect ratio, 17-inch rim.
- Enter the section width. This is the first number, in millimeters (e.g. 225).
- Enter the aspect ratio. This is the percentage of the width that equals the sidewall height. A 65 aspect ratio means the sidewall is 65% of 225mm = 146.25mm tall.
- Enter the rim diameter. This is the last number, in inches (e.g. 17).
- Read your results. Overall diameter is the full outside-to-outside measurement of the mounted tire. Revolutions per mile affects speedometer accuracy if you change tire sizes.
About the Tire Size Calculator
Overall diameter equals twice the sidewall height (converted to inches) plus the rim diameter. Sidewall height in mm equals width times aspect ratio divided by 100. Converting to inches: divide by 25.4. For a 225/65R17: sidewall is 225 times 0.65 = 146.25mm = 5.76 inches. Diameter equals (2 times 5.76) plus 17 = 28.52 inches. Circumference equals diameter times pi, and revolutions per mile equals 63,360 (inches per mile) divided by circumference. Changing tire diameter by more than 3% will affect speedometer accuracy and may trigger check engine lights on vehicles with electronic stability control.
Frequently Asked Questions
What happens to my speedometer if I change tire size?
Your speedometer reads based on wheel revolutions per mile. A larger tire makes fewer revolutions per mile than a smaller one, so the speedometer shows lower than actual speed. If you go from a 225/65R17 (28.52" diameter, 744 rev/mi) to a 265/70R17 (31.61" diameter, 672 rev/mi), your speedometer would read approximately 6% slow. Calculate both tire sizes and compare the revolutions per mile to quantify the error.
What is plus-sizing a tire?
Plus-sizing means increasing rim diameter while decreasing sidewall height to keep overall diameter nearly the same. Going from a 225/65R17 to a 225/55R18 keeps the diameter within about half an inch. The benefit is a wider, lower-profile look and often better handling from a stiffer sidewall. The drawback is a harsher ride and higher risk of rim damage on rough roads.
How do I know if a different tire size will fit my car?
The three critical clearances are: overall diameter (must not rub the wheel well at full suspension compression), section width (must clear the inner fender liner and suspension components), and load rating (must meet or exceed the original tire's load index). A 3% change in diameter and up to 20mm change in width is generally safe for most vehicles, but verify with a fitment guide for your specific make and model.
What does the load index number mean on a tire?
The load index is a number that corresponds to the maximum weight each tire can support at the specified speed rating. A load index of 91 means the tire can support 1,356 lbs. With four tires, that is 5,424 lbs total. Your replacement tires must have a load index equal to or higher than the original equipment tires. The load index is printed on the tire sidewall after the tire size and before the speed rating letter.