What is the diameter difference between 255/45R18 and 275/35R20?

255/45R18 has an overall diameter of 27.04 inches (687 mm). 275/35R20 is 27.58 inches (701 mm). That's a 0.54 inch (14 mm) difference — 275/35R20 is taller.

Will 275/35R20 cause speedometer error on a car fitted with 255/45R18?

Swapping 255/45R18 for 275/35R20 produces a speedometer error of about 2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.2 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/45R18 turns 746 revs per mile. 275/35R20 turns 731 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/45R18 has a sidewall of 115 mm (4.52 in). 275/35R20 sidewall is 96 mm (3.79 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/35R20 a safe plus-size upgrade from 255/45R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

255/45R18 vs 275/35R20

Overall diameter, sidewall, revs per mile, and speedometer error — 275/35R20 is the taller tire, with a 2% speedo delta when swapping.

255/45R18

27.04in

687 mm overall diameter

275/35R20

27.58in

701 mm overall diameter

Diameter Δ

0.54 in

Speedo error

255/45R18 revs/mi

746

275/35R20 revs/mi

731

Within the ±3% overall-diameter band usually considered safe for plus-sizing — speedometer and ABS calibration should stay close to factory.

Full spec

Side by side

Metric	255/45R18	275/35R20
Section width	255 mm	275 mm
Aspect ratio	45	35
Wheel diameter	18″	20″
Sidewall height	115 mm	96 mm
Overall diameter	27.04 in (687 mm)	27.58 in (701 mm)
Circumference	84.93 in	86.64 in
Revolutions / mile	746	731

Other comparisons for these sizes

235/45R18 vs 255/45R18 245/45R18 vs 255/45R18 255/40R18 vs 255/45R18 255/45R18 vs 255/40R19 255/35R20 vs 275/35R20 265/35R20 vs 275/35R20 275/30R20 vs 275/35R20 275/35R20 vs 285/30R21 275/35R20 vs 275/30R21 275/35R20 vs 285/35R20

FAQ

Frequently asked

What is the diameter difference between 255/45R18 and 275/35R20?: 255/45R18 has an overall diameter of 27.04 inches (687 mm). 275/35R20 is 27.58 inches (701 mm). That's a 0.54 inch (14 mm) difference — 275/35R20 is taller.
Will 275/35R20 cause speedometer error on a car fitted with 255/45R18?: Swapping 255/45R18 for 275/35R20 produces a speedometer error of about 2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.2 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/45R18 turns 746 revs per mile. 275/35R20 turns 731 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/45R18 has a sidewall of 115 mm (4.52 in). 275/35R20 sidewall is 96 mm (3.79 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/35R20 a safe plus-size upgrade from 255/45R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.