Tire Size Calculator
Compare two tire sizes and see the change in diameter, width, revolutions per mile and speedometer reading.
Results
| Measure | Current | New | Change |
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Speedometer check
| Speedometer shows | Actual speed |
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Quick Answer
A tire’s overall diameter equals the rim diameter plus two sidewall heights. For a 225/65R17 tire, each sidewall is 5.76 inches, so the overall diameter is 28.52 inches. Changing to 235/60R18 adds about 2.06%, so a speedometer showing 60 mph means you are really doing about 61.2 mph.
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Table of Contents
How This Tire Size Calculator Works
A tire size such as 225/65R17 has three main numbers: width in millimeters, aspect ratio, and rim diameter in inches. The calculator turns these into real measurements for both tires and shows the difference.
Quick explanation: Overall diameter = rim + 2 × sidewall height.
- Width: the first number, in millimeters.
- Aspect ratio: sidewall height as a percentage of width.
- Rim diameter: the wheel size, in inches.
Core Formulas
- Sidewall height (in): width × aspect ratio ÷ 100 ÷ 25.4
- Overall diameter (in): rim + 2 × sidewall height
- Circumference (in): π × overall diameter
- Revolutions per mile: 63,360 ÷ circumference
- Actual speed: speedometer reading × new diameter ÷ current diameter
Worked Example
Compare 205/55R16 with 215/45R17.
- 205/55R16 sidewall = 205 × 55 ÷ 100 ÷ 25.4 = 4.44 in, so diameter = 24.88 in
- 215/45R17 sidewall = 215 × 45 ÷ 100 ÷ 25.4 = 3.81 in, so diameter = 24.62 in
- Change = −1.04%, so 60 mph on the speedometer is about 59.4 mph
Before You Change Tire Size
- Check your door label or manual: these list approved sizes for your vehicle.
- Clearance: a larger tire can touch suspension or bodywork when turning.
- Load and speed ratings: a new tire must meet your vehicle’s requirements.
- Size change: ask a tire professional before fitting a size with a noticeably different diameter.
Frequently Asked Questions
In 225/65R17, 225 is the width in millimeters, 65 is the aspect ratio, R means radial construction, and 17 is the rim diameter in inches.
Multiply width by aspect ratio, divide by 100, then by 25.4 to get sidewall height in inches. Double it and add the rim diameter.
Yes. A larger diameter means the speedometer reads lower than your real speed. A smaller diameter makes it read higher.
It is the sidewall height as a percentage of the tire’s width. A 65 aspect ratio means the sidewall is 65% of the width.
Sometimes, but you should check your vehicle maker’s approved sizes and ask a tire professional. Clearance, load rating and speed rating all matter.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Bridgestone Americas, determining tire size | Meaning of tire size numbers. | https://www.bridgestoneamericas.com/en/company/safety/choosing-tires/determining-tire-size |
| Modern Tire Dealer, understanding tire dimensions | Aspect ratio definition. | https://www.moderntiredealer.com/articles/17231-understanding-tire-dimensions |