Tire Size Calculator
Compare two tire dimensions side-by-side to calculate differences in overall diameter, sidewall height, revs per mile, and speedometer error.
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How Tire Dimensions are Calculated
Metric tire codes (e.g. 225/45R17) provide width in millimeters, aspect ratio as a percentage, and rim diameter in inches:
Sidewall Height
Sidewall = (Width_mm × Aspect%) / 25.4
Total Diameter
Diameter = Rim_in + (2 × Sidewall_in)
The 3% Rule for Plus-Sizing Wheels
When upgrading to larger wheels or wider tires, automotive engineers recommend keeping overall tire diameter within ± 3% of the original factory specification. Exceeding 3% alters speedometer accuracy, ABS braking logic, traction control, and transmission shift points.
Frequently Asked Questions
What happens if my new tires are larger in diameter?
If new tires are taller, your speedometer will read slower than you are actually traveling (e.g. your speedometer reads 60 MPH when you are doing 62.5 MPH). It also slightly reduces low-end acceleration while lowering highway cruising engine RPM.
Does tire diameter change ride clearance height?
Yes. Ground clearance changes by exactly half of the total tire diameter difference (e.g. a 1-inch taller tire raises your vehicle by 0.5 inches).
What is revolutions per mile?
Revolutions per mile is the number of complete rotations a tire makes over 5,280 feet. It is calculated by dividing 63,360 inches (1 mile) by the tire circumference.