# Tire Size Comparison Calculator

> Tire size comparison calculator: compare two tire sizes for diameter difference, speedometer error, and fitment before you buy. Free, private, no signup.

- **Canonical URL:** https://boringtoolskit.com/tire-size-comparison-calculator/
- **Category:** Vehicles
- **Evidence level:** authored-methodology
- **Reviewed:** July 26, 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Diameter (in) = wheel + 2 x (sidewall), where sidewall = width x aspect ratio / 100 / 25.4; difference % = (d2 - d1) / d1 x 100.

## Worked example

Replacing 225/55R17 tires (26.74 in diameter) with 235/50R18 (27.25 in) grows the diameter by 1.9%.

## Assumptions

- Sizes follow the standard metric tire code: width in mm, aspect ratio as a percent of width.
- Speedometer error scales with the diameter difference.

## Common questions

### How much diameter change is safe?

Keep the overall diameter within about 3% of the original to protect speedometer, odometer, and fitment.

### Does a bigger wheel mean a bigger tire?

Not necessarily: plus-size swaps use lower-profile sidewalls to keep the diameter similar.

## Related tools

- /gas-mileage-calculator/
- /mpg-calculator/
- /fuel-cost-calculator/

## Sources

- [Tire safety information](https://www.nhtsa.gov/equipment/tires)
- [Calculation methodology](/methodology/)

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