Tire Size Calculator

Enter any metric tire size (e.g. 275/65R18) to see diameter, circumference, and sidewall height. Compare two sizes to estimate speedometer error before an upsize.

Type to search · try 275/65R18 or "33" for inch aliases

Current / Stock Size

Common upsizes from 265/70R17:

New / Comparison Size

Don't know your stock size? Load it by vehicle:

265/70R17

Diameter: 31.61″ / 802.8mm
Section Width: 10.43″
Sidewall: 7.3″ / 185.5mm
Circumference: 99.29″
Revs/Mile: 638

See tires in 265/70R17 →

275/65R18

Diameter: 32.07″ / 814.7mm
Section Width: 10.83″
Sidewall: 7.04″ / 178.75mm
Circumference: 100.77″
Revs/Mile: 629

See tires in 275/65R18 →

Speedometer & Diameter Impact

Different rim — new wheels required

Diameter Change

+1.48%

Sidewall Change

-0.26″

Revs/Mile Δ

-9

Indicated speed:mph

At an indicated 60 mph, your actual speed is 60.89 mph (97.99 km/h). Your speedometer reads 1.48% low versus actual speed.

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How this calculator works

A tire marked 275/65R18 has a 275 mm section width, a sidewall that is 65% of the width (178.75 mm), and mounts on an 18 inch rim. The overall diameter is rim + 2×sidewall, converted to inches. Speedometer error between two sizes is the ratio of their diameters — a taller tire makes the speedometer read low by that percentage.