What is the diameter difference between 255/35R21 and 275/35R21?

255/35R21 has an overall diameter of 28.03 inches (712 mm). 275/35R21 is 28.58 inches (726 mm). That's a 0.55 inch (14 mm) difference — 275/35R21 is taller.

Will 275/35R21 cause speedometer error on a car fitted with 255/35R21?

Swapping 255/35R21 for 275/35R21 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?

255/35R21 turns 720 revs per mile. 275/35R21 turns 706 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/35R21 has a sidewall of 89 mm (3.51 in). 275/35R21 sidewall is 96 mm (3.79 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/35R21 a safe plus-size upgrade from 255/35R21?

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

255/35R21 vs 275/35R21

Overall diameter, sidewall, revs per mile, and speedometer error — 275/35R21 is the taller tire, with a 2% speedo delta when swapping.

255/35R21

28.03in

712 mm overall diameter

275/35R21

28.58in

726 mm overall diameter

Diameter Δ

0.55 in

Speedo error

255/35R21 revs/mi

720

275/35R21 revs/mi

706

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	255/35R21	275/35R21
Section width	255 mm	275 mm
Aspect ratio	35	35
Wheel diameter	21″	21″
Sidewall height	89 mm	96 mm
Overall diameter	28.03 in (712 mm)	28.58 in (726 mm)
Circumference	88.05 in	89.78 in
Revolutions / mile	720	706

Other comparisons for these sizes

255/35R21 vs 265/35R21 265/35R21 vs 275/35R21 275/30R21 vs 275/35R21 275/35R21 vs 285/30R22 275/35R21 vs 295/35R21 275/35R21 vs 285/30R21

FAQ

Frequently asked

What is the diameter difference between 255/35R21 and 275/35R21?: 255/35R21 has an overall diameter of 28.03 inches (712 mm). 275/35R21 is 28.58 inches (726 mm). That's a 0.55 inch (14 mm) difference — 275/35R21 is taller.
Will 275/35R21 cause speedometer error on a car fitted with 255/35R21?: Swapping 255/35R21 for 275/35R21 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?: 255/35R21 turns 720 revs per mile. 275/35R21 turns 706 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/35R21 has a sidewall of 89 mm (3.51 in). 275/35R21 sidewall is 96 mm (3.79 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/35R21 a safe plus-size upgrade from 255/35R21?: 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.