Tire Size Calculator
Compare two tire sizes instantly. Check diameter differences and speedometer error.
1 Current Tire
2 New Tire
Comparison Results
Target calculated| Metric | Tire 1 | Tire 2 | Difference |
|---|---|---|---|
| Diameter | - | - | - |
| Width | - | - | - |
| Sidewall | - | - | - |
| Circumference | - | - | - |
| Revs/Mile | - | - | - |
Speedometer reading is accurate.
Compare Tire Sizes Before You Buy
Enter your current tire size and the size you are considering, and this free calculator shows the diameter, width, sidewall height, and circumference of each — plus the percentage difference that affects your speedometer.
Swapping tire sizes changes ride height, gearing, and speed readings. See the numbers first, then shop with confidence. Everything computes instantly in your browser.
Side-by-side dimensions
Overall diameter, section width, sidewall height, and circumference for both tires, shown together for easy comparison.
Speedometer difference
See exactly how much your speedometer will be off — and how your ride height changes — before committing to a new size.
Free and private
All tire math runs in your browser — nothing you enter is uploaded anywhere.
How to use
- Enter your current tire size (e.g. 205/55R16).
- Enter the new tire size you are comparing.
- Click Calculate to see the dimensions and speedometer difference.
Frequently Asked Questions
How do I read a tire size like 205/55R16?
205 is the width in mm, 55 is the sidewall as a percentage of width, R means radial, and 16 is the wheel diameter in inches.
Why does tire size affect my speedometer?
A taller tire covers more ground per revolution, so your speedometer — calibrated for the original size — reads low; a shorter tire reads high.
How much size difference is safe?
Staying within about 3% of the original diameter is the common rule of thumb, but always check your vehicle manufacturer's guidance.
Does changing tire size affect fuel economy?
It can — larger, heavier tires typically increase rolling resistance, which may slightly reduce fuel efficiency.
Is the calculator free?
Yes, free with no signup.
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