Free calculator
Tire Size Comparison
Compare two tire sizes and calculate diameter, circumference and speedometer error.
Formula diameter = wheel + 2 × sidewall; error (%) = (new diameter ÷ original diameter − 1) × 100
Compare tire diameter and speedometer difference
Calculate the nominal diameter of both sizes, then divide the replacement diameter by the original diameter. Subtract one and multiply by 100 for percentage difference. At the same wheel rotation rate, a larger replacement travels farther, so actual speed is higher than the reading calibrated for the original tire.
Worked example
225/50R17 is about 25.86 inches in diameter; 235/45R18 is about 26.33 inches. The difference is approximately +1.84%. At an indicated 60 mph, the diameter ratio gives about 61.1 mph, before other measurement effects.
How to use the result
The first tire is the original reference and the second is the replacement. Reversing them changes the direction and percentage. Matching diameter does not establish compatible width, offset, load rating or clearance.
Sources and assumptions
Calculation methodology. Worked examples use the expressly stated inputs; sample prices are illustrative rather than local quotes.
Use a vehicle service sheet next
Related calculators
Frequently asked questions
Do bigger tires make the speedometer read low?
With the original calibration unchanged, a larger rolling diameter travels farther per wheel revolution. The geometric estimate therefore gives a higher actual speed than indicated.
What does a negative tire-size error mean?
The second tire has a smaller nominal diameter than the first. It travels less distance per revolution, so the estimated actual speed is lower than indicated.
Is a small percentage difference automatically safe?
No. This tool reports geometry only. Vehicle, wheel, clearance and load requirements still need to be checked.
Last updated