Tire Size & Speed Comparison

When to use it

For comparing the geometry encoded by two common metric tire sizes, such as 205/55R16 and 225/45R17. A is the original reference and B the alternative. Keep the codes and the calculation together; this is not a vehicle-fitment approval.

Inputs

Calculation method

Sidewall height = section width × aspect ratio. Diameter = rim diameter + twice the sidewall height, converting inches and millimetres at 1 in = 25.4 mm. Compare B with A, then multiply indicated speed by diameter B ÷ diameter A. The codes in this example give nominal diameters of 631.9 and 634.3 mm, a difference of 2.4 mm or about 0.38%. At an indicated 60 mph, the unchanged-calibration estimate is about 60.23 mph.

205 [Original section width, code A mm] × 55 [Original aspect ratio, code A]% → Original nominal sidewall height mm

225 [Alternative section width, code B mm] × 45 [Alternative aspect ratio, code B]% → Alternative nominal sidewall height mm

16 [Original rim diameter, code A in] + 2 × Original nominal sidewall height mm → Original nominal outside diameter mm

17 [Alternative rim diameter, code B in] + 2 × Alternative nominal sidewall height mm → Alternative nominal outside diameter mm

Alternative nominal outside diameter mm-Original nominal outside diameter mm → Alternative minus original nominal diameter mm

Alternative nominal outside diameter mm/Original nominal outside diameter mm-1 → Relative change from the original diameter

60 [Reference indicated speed with the original calibration mph] × Alternative nominal outside diameter mm/Original nominal outside diameter mm → Estimated speed under the unchanged original-calibration assumption mph

Change one input

Change only profile B from 45% to 50%. Its nominal sidewall becomes 112.5 mm and diameter 656.8 mm. Diameter difference becomes 24.9 mm, about 3.94%, and the speed estimate at the same indicated 60 mph becomes about 62.36 mph.

Results are displayed to at most two decimal places; later formulas use unrounded values.

Scope and limits

• A is the original tire; B is the comparison tire.
• Codes mix mm, aspect ratio and inches in every language.
• Nominal geometry does not establish vehicle fitment or measured road speed.

Widths and rim diameters must be positive, aspect ratios greater than 0 and at most 100%, and reference speed nonnegative. The estimate assumes the original tire defined the calibration exactly and uses nominal diameter as a rolling-size approximation. It does not account for tire pressure, load, wear, actual rolling circumference or the vehicle’s original speedometer offset. No flotation sizing, wheel width/offset, clearance, load rating, speed rating or installation legality is checked; no ±3% rule is used as permission to fit a tire.

Nominal tire dimensions: Tire Rack.

Download and import into Calvance to edit the numbers. The canvas does not reject every invalid input; check the stated ranges.

Common questions

Does a small diameter difference mean the tire fits?

No. Vehicle clearance, wheels, load and speed requirements need separate confirmation.

Is the speed result a measurement?

No. It isolates the effect of a nominal diameter change under an unchanged calibration; actual road speed can differ.

How to import your template

We recommend using the latest version of Calvance.

1. Save the file. Choose Download template and save the .calvx file to Files on your iPhone or iPad. From a computer, you can send it to your device with AirDrop.
2. Open Import. In All Canvases in Calvance, tap the add button to open the template screen, then choose Import.
3. Choose the file. Select the saved .calvx file to create your own canvas. Importing uses one canvas from your allowance.
4. Edit and check. Check the results against the worked example, then add your numbers. Duplicate the canvas to keep different versions.

Download .calvx file