What is the diameter difference between 275/30R21 and 275/35R21?

275/30R21 has an overall diameter of 27.5 inches (698 mm). 275/35R21 is 28.58 inches (726 mm). That's a 1.08 inch (28 mm) difference — 275/35R21 is taller.

Will 275/35R21 cause speedometer error on a car fitted with 275/30R21?

Swapping 275/30R21 for 275/35R21 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.4 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

275/30R21 turns 733 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?

275/30R21 has a sidewall of 83 mm (3.25 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 275/30R21?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.9%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

275/30R21 vs 275/35R21

Overall diameter, sidewall, revs per mile, and speedometer error — 275/35R21 is the taller tire, with a 3.9% speedo delta when swapping.

275/30R21

27.5in

698 mm overall diameter

275/35R21

28.58in

726 mm overall diameter

Diameter Δ

1.08 in

Speedo error

3.9%

275/30R21 revs/mi

733

275/35R21 revs/mi

706

Outside the typical ±3% safe plus-size band — you may want speedometer recalibration, and this change can affect ABS or traction control behaviour.

Full spec

Side by side

Metric	275/30R21	275/35R21
Section width	275 mm	275 mm
Aspect ratio	30	35
Wheel diameter	21″	21″
Sidewall height	83 mm	96 mm
Overall diameter	27.5 in (698 mm)	28.58 in (726 mm)
Circumference	86.38 in	89.78 in
Revolutions / mile	733	706

Other comparisons for these sizes

255/40R19 vs 275/30R21 265/35R20 vs 275/30R21 275/35R20 vs 275/30R21 255/35R21 vs 275/35R21 265/35R21 vs 275/35R21 265/35R21 vs 275/30R21 275/30R21 vs 285/30R21 275/30R21 vs 295/30R21 275/35R21 vs 285/30R22 275/35R21 vs 295/35R21

FAQ

Frequently asked

What is the diameter difference between 275/30R21 and 275/35R21?: 275/30R21 has an overall diameter of 27.5 inches (698 mm). 275/35R21 is 28.58 inches (726 mm). That's a 1.08 inch (28 mm) difference — 275/35R21 is taller.
Will 275/35R21 cause speedometer error on a car fitted with 275/30R21?: Swapping 275/30R21 for 275/35R21 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.4 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 275/30R21 turns 733 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?: 275/30R21 has a sidewall of 83 mm (3.25 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 275/30R21?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.9%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.