How to read a tire size
In 245/40R18, 245 is the section width in millimeters, 40 is the sidewall height as a percentage of width (98 mm here), R means radial, and 18 is the wheel diameter in inches. Overall diameter = rim × 25.4 + 2 × sidewall.
Tire Size Calculator & Comparison
Enter two tire sizes to compare diameter, sidewall, width and revolutions per mile, and see how much your speedometer changes.
Compare two tire sizes
Result
| Current | New | Difference |
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Calculated from the size code. Real tires vary a little by brand and tread depth.
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The 3% rule
Many shops aim to keep overall diameter within about 3% of stock to limit speedometer error and avoid ABS or traction-control issues. Going wider? Check the offset change with the wheel offset calculator. For indicated speed and RPM after the swap, use the speedometer error calculator and the gear ratio & RPM calculator.
Will a bigger tire make my speedometer wrong?
Yes. A taller tire covers more ground per turn, so the speedometer reads lower than your real speed. The calculator shows your real speed at an indicated 60 mph.
Can I put a wider tire on my current wheel?
Often by one size (about 10 mm), but check the tire maker's approved rim width range for that size.