# Tire Size Calculator

Calculate tire overall diameter, sidewall height, circumference, and revolutions per mile from standard tire size numbers (e.g. 225/45R17).

## What this calculates

Decode those confusing tire size numbers into real dimensions. Enter the three numbers from your tire size (like 225/45R17) to calculate overall diameter, sidewall height, circumference, and revolutions per mile. Use this to compare alternative tire sizes.

## Inputs

- **Tire Width** (mm) — min 100, max 400 — First number in tire size (e.g. 225 in 225/45R17)
- **Aspect Ratio** (%) — min 20, max 100 — Second number (e.g. 45 in 225/45R17)
- **Wheel Diameter** (inches) — min 10, max 30 — Last number (e.g. 17 in 225/45R17)

## Outputs

- **Overall Diameter** (in)
- **Overall Diameter**
- **Circumference** (in)
- **Sidewall Height** (mm)
- **Revolutions per Distance** — formatted as text — Wheel revolutions per unit distance.

## Details

Tire sizes follow the format: Width/AspectRatio R WheelDiameter

  - Width (mm): The tire's cross-section width

  - Aspect Ratio (%): Sidewall height as a percentage of width

  - R: Radial construction

  - Wheel Diameter (inches): The wheel rim size

So a 225/45R17 tire is 225mm wide, with a sidewall that is 45% of 225mm (101.25mm), mounted on a 17-inch wheel.

Tip: When changing tire sizes, keep the overall diameter within 3% of the original to avoid speedometer inaccuracy and clearance issues.

## Frequently Asked Questions

**Q: What does the tire size 225/45R17 mean?**

A: The 225 is the tire width in millimeters. The 45 is the aspect ratio, meaning the sidewall height is 45% of the width. R means radial construction. 17 is the wheel diameter in inches. So the sidewall is 101.25mm tall.

**Q: How much can I change tire size without issues?**

A: Keep the overall diameter within 3% of the original. Larger changes can affect speedometer accuracy, ABS, traction control, and may cause rubbing in the wheel well. A 3% change causes roughly 3% speedometer error.

**Q: Does tire size affect gas mileage?**

A: Yes. Wider tires have more rolling resistance and can reduce fuel economy by 1-3%. Taller tires change the effective gear ratio. Larger tires are also heavier, requiring more energy to accelerate.

**Q: What does the speed rating on a tire mean?**

A: The speed rating (a letter like H, V, W, Y after the size) indicates the maximum safe speed: H=130 mph, V=149 mph, W=168 mph, Y=186 mph, Z=149+ mph. Always match or exceed your vehicle's recommended rating.

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Source: https://vastcalc.com/calculators/everyday/tire-size
Category: Everyday Life
Last updated: 2026-04-21