Why Use This Calculator?
A tire size like 225/65R17 isn't random — width in millimeters, sidewall as a percentage of that width, and rim diameter in inches. The calculator decodes it into the numbers that matter: overall diameter, sidewall height, and how many times the tire turns per mile.
The optional second size is the real power: swapping to a different profile changes the rolling diameter, which makes your speedometer read wrong and can rub the wheel well. The tool flags any swap outside the safe 3% band.
How It Works
- Enter your original tire size from the sidewall or door sticker.
- Optionally enter a replacement size you're considering.
- Check the diameter and speedometer error before buying anything.
Frequently Asked Questions
What do tire size numbers mean?
225/65R17 means 225 mm section width, sidewall height at 65% of that width, and a 17-inch rim. Overall diameter is the rim plus twice the sidewall — the calculator does the unit conversion.
How much speedometer error is safe with different tires?
Stay within about 3% of the original diameter. Beyond that, the speedometer reads wrong, odometer accuracy drifts, and larger tires can rub suspension components at full lock.
Where do I find my original tire size?
It's molded into the sidewall (e.g., 225/65R17) and printed on the driver's door-jamb sticker — the door sticker is the safest source for the factory-correct size.
Tips & Things to Watch
- Plus-sizing wheels usually means lower profile tires — keep the total diameter matched.
- Bigger diameter = fewer revs per mile = odometer under-reports and speedo over-reads.
- Winter tires in the original size avoid every clearance and calibration question.