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: 265/40R19 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

−5.96% 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 19″ rims.
20 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	265/40R19	245/40R18	Diff	%
Overall diameter	27.35″695 mm	25.72″653 mm	−1.63″	−6.0%
Section width	265.0 mm	245.0 mm	−20.0 mm	−7.5%
Sidewall height	106.0 mm	98.0 mm	−8.0 mm	−7.5%
Rim diameter	19″	18″	−1″	−5.3%
Circumference	85.91″2182 mm	80.79″2052 mm	−5.12″	−6.0%
Revolutions per mileturns per distance; drives the odometer	738458 /km	784487 /km	+47	+6.3%
Ride height change	—	—	−0.81″ −20.7 mm

Speedometer reading

Speedo shows

60mph

97 km/h

Actual speed

56.4mph

91 km/h

Overlay

265/40R19245/40R18

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.