Tire Size Calculator

A tire size calculator compares two P-metric tire sizes and shows how they differ in overall diameter, circumference, and sidewall height — and most importantly, how a change will affect your speedometer reading. It's the first tool to reach for whenever you're considering plus-sizing wheels, fitting larger off-road tires, or replacing OEM tires with a different fitment.

Tire sizes use a notation like 225/45R17: 225 mm tread width, 45% aspect ratio, radial construction, 17-inch rim. Two tire sizes that look similar on paper can differ by half an inch in overall diameter, which is enough to throw off your speedometer, ABS calibration, and even the transmission's shift points on a modern car.

Comparing OEM 215/55R17 against a plus-1 upgrade to 225/45R18:

• 215/55R17 sidewall: 215 × 0.55 = 118.25 mm; diameter ≈ 26.31 in
• 225/45R18 sidewall: 225 × 0.45 = 101.25 mm; diameter ≈ 25.97 in
• Diameter difference: −1.3% (the new setup is 0.34 in smaller)
• Speedometer error: reads about 1.3% fast — at an indicated 60 mph, true speed is ~59.2 mph

That's well within the 3% tolerance most manufacturers allow, so it's a safe swap. A jump to 265/70R17 off-road tires (30.6 in diameter) would be a 16% increase — far too large without re-gearing and recalibration.

• Plus-sizing wheels — verify the new tire keeps overall diameter within 3% of OEM.
• Fitting larger off-road tires — see how much speedometer recalibration you'll need.
• Buying winter tires on smaller rims — many drivers downsize one rim size to fit cheaper, taller-sidewall snow tires.
• Diagnosing speedometer error — compare your actual installed tires against the OEM size to explain a wrong reading.
• Mixing staggered fitment — confirm front and rear tires have the same circumference if your AWD/4WD car requires it.

• Ignoring load index and speed rating. A correctly-sized tire that lacks the original's load rating is unsafe — always match or exceed the OEM specs.
• Going more than 3% larger without a tuner. ABS, traction control, and odometer-driven service intervals all drift out of spec.
• Forgetting fender clearance. Diameter math says it fits, but real-world steering and suspension travel can cause rubbing on lock or compression.
• Assuming all 215/55R17 tires are identical. Manufacturer tolerances vary by ±5 mm; check the brand's actual section width and revolutions-per-mile spec.
• Not recalibrating the speedometer after a big change. On modern OBD-II cars, a $50 calibration tool fixes the reading and protects warranty in some cases.