What is the diameter difference between 275/35R21 and 285/30R21?

275/35R21 has an overall diameter of 28.58 inches (726 mm). 285/30R21 is 27.73 inches (704 mm). That's a 0.85 inch (22 mm) difference — 285/30R21 is shorter.

Will 285/30R21 cause speedometer error on a car fitted with 275/35R21?

Swapping 275/35R21 for 285/30R21 produces a speedometer error of about 3%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.2 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

275/35R21 turns 706 revs per mile. 285/30R21 turns 727 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

275/35R21 has a sidewall of 96 mm (3.79 in). 285/30R21 sidewall is 86 mm (3.37 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 285/30R21 a safe plus-size upgrade from 275/35R21?

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

Tire size comparison

275/35R21 vs 285/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 275/35R21 is the taller tire, with a 3% speedo delta when swapping.

275/35R21

28.58in

726 mm overall diameter

285/30R21

27.73in

704 mm overall diameter

Diameter Δ

0.85 in

Speedo error

275/35R21 revs/mi

706

285/30R21 revs/mi

727

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	275/35R21	285/30R21
Section width	275 mm	285 mm
Aspect ratio	35	30
Wheel diameter	21″	21″
Sidewall height	96 mm	86 mm
Overall diameter	28.58 in (726 mm)	27.73 in (704 mm)
Circumference	89.78 in	87.12 in
Revolutions / mile	706	727

Other comparisons for these sizes

275/35R20 vs 285/30R21 285/35R20 vs 285/30R21 255/35R21 vs 275/35R21 265/35R21 vs 275/35R21 275/30R21 vs 285/30R21 275/30R21 vs 275/35R21 275/35R21 vs 285/30R22 275/35R21 vs 295/35R21 285/30R21 vs 295/30R21 285/30R21 vs 305/30R21

FAQ

Frequently asked

What is the diameter difference between 275/35R21 and 285/30R21?: 275/35R21 has an overall diameter of 28.58 inches (726 mm). 285/30R21 is 27.73 inches (704 mm). That's a 0.85 inch (22 mm) difference — 285/30R21 is shorter.
Will 285/30R21 cause speedometer error on a car fitted with 275/35R21?: Swapping 275/35R21 for 285/30R21 produces a speedometer error of about 3%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.2 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 275/35R21 turns 706 revs per mile. 285/30R21 turns 727 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 275/35R21 has a sidewall of 96 mm (3.79 in). 285/30R21 sidewall is 86 mm (3.37 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 285/30R21 a safe plus-size upgrade from 275/35R21?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.