Tire Size Calculator
A P-metric tire size like 225/45R17 packs three measurements into one code: 225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches. This tire size calculator decodes that string into the numbers you actually need — sidewall height, overall diameter, circumference, and revolutions per mile — so you can judge fitment before you buy.
It also runs a side-by-side comparison: enter your current (stock) size and the size you are considering, and you will see the diameter difference in inches and percent, how far your speedometer will be off at 60 mph, and whether the swap passes the industry-standard ±3% overall-diameter guideline. That rule matters more than most drivers realize — a taller tire makes your speedometer read slower than you are actually going, and oversized tires can rub fenders, confuse ABS and traction control calibration, and throw off odometer-based maintenance intervals. Stay within 3% and everything from gearing to stability control behaves as the engineers intended.
Format: width / aspect ratio R wheel diameter — e.g. 205/55R16. An optional P or LT prefix is fine (P225/45R17, LT265/70R17).
How to Use This Tire Size Calculator
- Find your size on the tire sidewall or the driver-door jamb sticker — it looks like 225/45R17.
- Enter your current size in the first box to see its full dimensions.
- Enter the size you are considering in the second box to compare.
- Check the verdict: green means the swap is within the ±3% diameter guideline; red means expect side effects.
- Mind the speedometer line: if your new tire is taller, your real speed is higher than indicated.
Formula and Assumptions
- Sidewall height = width (mm) × aspect ratio ÷ 100
- Overall diameter (in) = wheel diameter + 2 × sidewall ÷ 25.4
- Circumference (in) = π × diameter
- Revolutions per mile = 63,360 (inches in a mile) ÷ circumference
- Speedometer error: actual speed = indicated speed × (new diameter ÷ old diameter) — speedometers are calibrated to the stock tire
- The ±3% rule: tire industry guideline keeping replacement tires within 3% of stock overall diameter so ABS, traction control, and gearing stay in spec
- Revs per mile is a static geometric figure; real-world values run ~2–3% lower due to tread squish under load
FAQ
What does 225/45R17 actually mean?
225 mm tread width, a sidewall that is 45% as tall as the width (101.25 mm), radial construction (R), and a 17-inch wheel. Overall diameter works out to 24.97 inches.
Can I put a different size tire on my car?
Usually yes, if overall diameter stays within ±3% of stock, the rim width suits the tire, and nothing rubs at full steering lock or full suspension compression. Wider tires also need adequate rim width.
How does a bigger tire affect my speedometer?
A taller tire covers more ground per revolution, so at any indicated speed you are going faster. A tire 3% taller means your speedometer reads 60 mph while you actually travel 61.8 mph.
Will plus-sizing my wheels hurt my ride?
Going from 17-inch to 18-inch wheels with a lower-profile tire keeps overall diameter constant but shortens the sidewall — sharper steering response, but a firmer ride and greater rim damage risk on potholes.
Do all four tires need to match?
Yes for AWD vehicles — mismatched diameters force the center differential to slip constantly and can destroy it. On 2WD cars, matching per axle is the minimum safe practice.