Tire Size Calculator
Changing tire size changes the diameter of the wheel — and that shifts your speedometer, odometer and gearing. Compare two sizes in the standard width/aspect R rim notation to see the diameter difference and the speedometer error.
Your numbers
Result
- Current diameterinches
- 24.88
- New diameterinches
- 24.97
| Measure | Current tire | New tire |
|---|---|---|
| Sidewall height | 4.44 in | 3.99 in |
| Overall diameter | 24.88 in | 24.97 in |
| Diameter | 632 mm | 634 mm |
How to read a tire size
A tire marked 205/55R16 means: 205 mm wide, a sidewall that is 55% of the width, on a 16-inch rim. Overall diameter is rim (in mm) + 2 × sidewall. The sidewall of a 205/55 is 205 × 0.55 = 112.75 mm, giving a diameter of about 24.9 inches. This calculator does the arithmetic and shows the result in inches and millimeters.
Why your speedometer changes
The speedometer is calibrated for the original diameter. A larger tire travels farther per revolution, so you are actually going faster than the dash shows; a smaller tire does the reverse. If the new diameter is 3% larger, then at 60 mph indicated you are really doing about 62 mph. Staying within roughly 3% of the original diameter is the common rule of thumb.
Plus sizing and fitment
"Plus sizing" — a wider tire on a bigger rim with a lower profile — keeps the overall diameter about the same while changing the look and handling. Straying too far affects clearance in the wheel well, ride comfort, fuel economy and the accuracy of your instruments. Stick to sizes approved for your vehicle (owner's manual and the door-jamb placard).
Frequently asked questions
Sources
- Tire and Rim Association — tire size designation standards
- Vehicle tire placard and owner’s manual (approved sizes)
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