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Tires & wheels

Tire Size Comparison Calculator

Compare two tire sizes with a scale diagram, dimension table, and speedometer estimates in mph or km/h.

By AutomotiveCalc · Published September 9, 2026
How this calculator is checked

Automated checks cover formula examples and input validation. This page has not received an independent automotive professional review.

Sources & calculation method · Review standards · Report a problem

Your numbers

Example calculation

Example results are shown below. Controls become available when the calculator loads.

Choose mph or km/h for the speedometer estimate. Changing the unit reinterprets the speed number; enter your indicated speed in the selected unit.

Have a size code? Apply it here, or edit the dimensions below. Supports R or ZR metric sizes, including 225/45R17 91W and 225/45ZR17 (94Y) XL. Only dimensions are applied; load, speed and XL/RF markings are not evaluated. Size codes do not confirm vehicle fitment. How to read tire sizes →

Diameter difference1.42 %
Estimated actual speed60.85 mph
Original diameter24.97 in
New diameter25.33 in
Nominal tire diameter comparisonOriginal diameter 24.97 inches. New diameter 25.33 inches. Both circles use the same scale. Inner circles show wheel diameter.OriginalNew
Unloaded geometry at the same scale. Dashed circles show the wheels. Actual tire dimensions can differ.
Dimensions compared
MeasurementOriginalNew
Diameter (mm)634.30643.30
Sidewall (mm)101.25105.75
Wheel (in)17.0017.00
Speedometer comparison — assuming original calibration was accurate
Indicated (mph)Estimated actual (mph)
30.0030.43
45.0045.64
60.0060.85
70.0070.99

This comparison does not establish safe fitment, clearance, load capacity, or compatibility with vehicle electronics. Use manufacturer-approved specifications.

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How to use this calculator

Replace the example values with your own measurements or specifications. Keep the units shown beside each label. Results update as you type; reset restores the example.

Assumes calibration matches the original tire and is not changed. This is an illustrative geometry comparison, not approval of a size change. Do not infer safe fitment from a percentage threshold.

The formula

Diameter ratio = new diameter ÷ original diameter. Difference (%) = (ratio − 1) × 100. Actual speed ≈ indicated speed × ratio.

A worked example

Changing 225/45R17 to 235/45R17 increases nominal diameter by about 1.42%. An indicated 60 mph becomes approximately 60.85 mph if the speedometer was accurate with the original tires. The same ratio applies in km/h: an indicated 100 km/h becomes approximately 101.42 km/h.

Common questions

Will larger tires make the speedometer read low?

Under this model, yes: a larger rolling circumference travels farther per revolution. Factory calibration and actual rolling radius can alter the result.

Does this calculate clearance?

No. Suspension, offset, wheel width, bodywork, and steering geometry are not modeled.

Can I use km/h instead of mph?

Yes. Choose km/h, then enter your indicated speed. The result and speedometer table use that unit. Tire-diameter percentage change is independent of speed units. The table contains example speeds, not legal limits or driving recommendations.

Sources & calculation method

Related guidance: NHTSA TireWise: tire sizes and safety. Accessed September 9, 2026. The equation above describes this tool’s model; the reference provides practical context and does not endorse this site.

Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.