What is the diameter difference between 255/30R20 and 275/30R20?

255/30R20 has an overall diameter of 26.02 inches (661 mm). 275/30R20 is 26.5 inches (673 mm). That's a 0.47 inch (12 mm) difference — 275/30R20 is taller.

Will 275/30R20 cause speedometer error on a car fitted with 255/30R20?

Swapping 255/30R20 for 275/30R20 produces a speedometer error of about 1.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/30R20 turns 775 revs per mile. 275/30R20 turns 761 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/30R20 has a sidewall of 77 mm (3.01 in). 275/30R20 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/30R20 a safe plus-size upgrade from 255/30R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

255/30R20 vs 275/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 275/30R20 is the taller tire, with a 1.8% speedo delta when swapping.

255/30R20

26.02in

661 mm overall diameter

275/30R20

26.5in

673 mm overall diameter

Diameter Δ

0.47 in

Speedo error

1.8%

255/30R20 revs/mi

775

275/30R20 revs/mi

761

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	255/30R20	275/30R20
Section width	255 mm	275 mm
Aspect ratio	30	30
Wheel diameter	20″	20″
Sidewall height	77 mm	83 mm
Overall diameter	26.02 in (661 mm)	26.5 in (673 mm)
Circumference	81.76 in	83.24 in
Revolutions / mile	775	761

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 255/30R20 and 275/30R20?: 255/30R20 has an overall diameter of 26.02 inches (661 mm). 275/30R20 is 26.5 inches (673 mm). That's a 0.47 inch (12 mm) difference — 275/30R20 is taller.
Will 275/30R20 cause speedometer error on a car fitted with 255/30R20?: Swapping 255/30R20 for 275/30R20 produces a speedometer error of about 1.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/30R20 turns 775 revs per mile. 275/30R20 turns 761 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/30R20 has a sidewall of 77 mm (3.01 in). 275/30R20 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/30R20 a safe plus-size upgrade from 255/30R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.