What is the diameter difference between 275/35R20 and 275/30R21?

275/35R20 has an overall diameter of 27.58 inches (701 mm). 275/30R21 is 27.5 inches (698 mm). That's a 0.08 inch (2 mm) difference — 275/30R21 is shorter.

Will 275/30R21 cause speedometer error on a car fitted with 275/35R20?

Swapping 275/35R20 for 275/30R21 produces a speedometer error of about 0.3%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 59.8 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

275/35R20 turns 731 revs per mile. 275/30R21 turns 733 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

275/35R20 has a sidewall of 96 mm (3.79 in). 275/30R21 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/30R21 a safe plus-size upgrade from 275/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

275/35R20 vs 275/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 275/35R20 is the taller tire, with a 0.3% speedo delta when swapping.

275/35R20

27.58in

701 mm overall diameter

275/30R21

27.5in

698 mm overall diameter

Diameter Δ

0.08 in

Speedo error

0.3%

275/35R20 revs/mi

731

275/30R21 revs/mi

733

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	275/35R20	275/30R21
Section width	275 mm	275 mm
Aspect ratio	35	30
Wheel diameter	20″	21″
Sidewall height	96 mm	83 mm
Overall diameter	27.58 in (701 mm)	27.5 in (698 mm)
Circumference	86.64 in	86.38 in
Revolutions / mile	731	733

Other comparisons for these sizes

255/45R18 vs 275/35R20 255/40R19 vs 275/30R21 255/35R20 vs 275/35R20 265/35R20 vs 275/30R21 265/35R20 vs 275/35R20 275/30R20 vs 275/35R20 275/35R20 vs 285/30R21 275/35R20 vs 285/35R20 275/35R20 vs 295/35R20 275/35R20 vs 285/30R20

FAQ

Frequently asked

What is the diameter difference between 275/35R20 and 275/30R21?: 275/35R20 has an overall diameter of 27.58 inches (701 mm). 275/30R21 is 27.5 inches (698 mm). That's a 0.08 inch (2 mm) difference — 275/30R21 is shorter.
Will 275/30R21 cause speedometer error on a car fitted with 275/35R20?: Swapping 275/35R20 for 275/30R21 produces a speedometer error of about 0.3%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 59.8 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 275/35R20 turns 731 revs per mile. 275/30R21 turns 733 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 275/35R20 has a sidewall of 96 mm (3.79 in). 275/30R21 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/30R21 a safe plus-size upgrade from 275/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.