Tire Size Calculator
Compare two tire sizes — calculate overall diameter, sidewall height, circumference, and speedometer error.
Common Tire Size Equivalents
| Metric Size | Overall Diameter | Sidewall | Circumference |
|---|---|---|---|
| 195/65R15 | 635 mm / 25.0" | 127 mm | 1,994 mm |
| 205/55R16 | 632 mm / 24.9" | 113 mm | 1,985 mm |
| 225/45R17 | 634 mm / 25.0" | 101 mm | 1,991 mm |
| 245/40R18 | 643 mm / 25.3" | 98 mm | 2,020 mm |
| 265/35R20 | 666 mm / 26.2" | 93 mm | 2,092 mm |
Frequently Asked Questions
How do I read a tire size?
A standard tire size like 225/45R17 means: 225 = tread width in millimetres; 45 = aspect ratio (sidewall height as % of width, so 225 × 0.45 = 101.25 mm sidewall); R = radial construction; 17 = rim diameter in inches. Overall diameter = rim diameter × 25.4 + 2 × sidewall height.
What happens if I put different size tires on my car?
Changing tire size affects: speedometer accuracy (larger tires rotate fewer times per mile, making your speedometer read low), ground clearance, handling and cornering, and in some cases ABS and traction control calibration. A 3% or larger diameter change can cause issues. Minor size changes (within ±3%) are usually acceptable.
How much does tire size affect speedometer?
Speedometer error % = (original circumference / new circumference − 1) × 100. If your new tires are 3% larger in circumference, your speedometer reads about 3% low — if it shows 60 mph, your actual speed is 61.8 mph. This also affects odometer readings, trip computers, and mileage warranty calculations.
What is a plus-sizing upgrade?
Plus-sizing means increasing the rim diameter while decreasing the aspect ratio to keep the overall tire diameter the same (or nearly). For example, upgrading from 225/60R16 to 225/50R17 keeps a similar overall diameter while fitting a larger wheel. This improves handling and aesthetics but typically reduces ride comfort due to the shorter sidewall.
Can I use winter tires of a different size?
Yes — running a narrower tyre on a smaller rim for winter is common and beneficial. Narrower tires apply more pressure per unit area, improving snow traction. For example, summer 225/45R18 wheels might pair with 205/55R16 dedicated winter tires (keeping the rim diameter smaller reduces cost and allows narrower widths).