What is the diameter difference between 285/35R20 and 285/30R21?

285/35R20 has an overall diameter of 27.85 inches (708 mm). 285/30R21 is 27.73 inches (704 mm). That's a 0.12 inch (3 mm) difference — 285/30R21 is shorter.

Will 285/30R21 cause speedometer error on a car fitted with 285/35R20?

Swapping 285/35R20 for 285/30R21 produces a speedometer error of about 0.4%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 59.7 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

285/35R20 turns 724 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?

285/35R20 has a sidewall of 100 mm (3.93 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 285/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

285/35R20 vs 285/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 285/35R20 is the taller tire, with a 0.4% speedo delta when swapping.

285/35R20

27.85in

708 mm overall diameter

285/30R21

27.73in

704 mm overall diameter

Diameter Δ

0.12 in

Speedo error

0.4%

285/35R20 revs/mi

724

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	285/35R20	285/30R21
Section width	285 mm	285 mm
Aspect ratio	35	30
Wheel diameter	20″	21″
Sidewall height	100 mm	86 mm
Overall diameter	27.85 in (708 mm)	27.73 in (704 mm)
Circumference	87.51 in	87.12 in
Revolutions / mile	724	727

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 285/35R20 and 285/30R21?: 285/35R20 has an overall diameter of 27.85 inches (708 mm). 285/30R21 is 27.73 inches (704 mm). That's a 0.12 inch (3 mm) difference — 285/30R21 is shorter.
Will 285/30R21 cause speedometer error on a car fitted with 285/35R20?: Swapping 285/35R20 for 285/30R21 produces a speedometer error of about 0.4%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 59.7 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 285/35R20 turns 724 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?: 285/35R20 has a sidewall of 100 mm (3.93 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 285/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.