Tire size calculator

Enter your current size and the size you’re considering. Everything below updates as you change the numbers — diameter, clearance, speedometer error and whether it fits.

Example: 255/60R15 vs 245/40R18. Change either size to compare your own.

Start from your car make, model, year

Make Model Year

Width/AspectRRim

DiameterxWidthRRim

Width/AspectRRim

DiameterxWidthRRim

Width in mm · aspect = sidewall height as a % of width · rim in inches

Flotation (LT) sizes, all in inches: overall diameter × section width, then rim

Outside the 3% rule

−4.92% diameter

This exceeds the tolerance most manufacturers allow. Expect speedometer error, possible rubbing, and on some cars ABS or stability-control faults.

Needs 18″ wheels: this tire does not fit your 15″ rims.
10 mm narrower: check the tire is approved for your rim width.

Check what fits your car Factory tire sizes for every make, model and year.

Tires in 245/40R18

Check price on Amazon · 245/40R18 Best-rated 245/40R18 tires

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Measurement	255/60R15	245/40R18	Diff	%
Overall diameter	27.05″687 mm	25.72″653 mm	−1.33″	−4.9%
Section width	255.0 mm	245.0 mm	−10.0 mm	−3.9%
Sidewall height	153.0 mm	98.0 mm	−55.0 mm	−35.9%
Rim diameter	15″	18″	+3″	+20.0%
Circumference	84.97″2158 mm	80.79″2052 mm	−4.18″	−4.9%
Revolutions per mileturns per distance; drives the odometer	746463 /km	784487 /km	+39	+5.2%
Ride height change	—	—	−0.67″ −16.9 mm

Speedometer reading

Speedo shows

60mph

97 km/h

Actual speed

57.0mph

92 km/h

Overlay

255/60R15245/40R18

How to use the tire size calculator

Put your current tire size — width, aspect ratio and rim diameter — into the first form, then your desired size into the second. You’ll get the difference between the new and the old size, plus the speedometer reading difference and your actual speed on the new set. Both metric and flotation (LT inch) sizes are supported.

The calculator helps you decide whether a set of tires and wheels will fit your car. To compare tires by brand and model instead, use our tire comparison tool; to look up factory sizes, browse by car make.