What is the diameter difference between 255/30R20 and 255/35R20?

255/30R20 has an overall diameter of 26.02 inches (661 mm). 255/35R20 is 27.03 inches (687 mm). That's a 1 inch (26 mm) difference — 255/35R20 is taller.

Will 255/35R20 cause speedometer error on a car fitted with 255/30R20?

Swapping 255/30R20 for 255/35R20 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/30R20 turns 775 revs per mile. 255/35R20 turns 746 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/30R20 has a sidewall of 77 mm (3.01 in). 255/35R20 sidewall is 89 mm (3.51 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/35R20 a safe plus-size upgrade from 255/30R20?

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

255/30R20 vs 255/35R20

Overall diameter, sidewall, revs per mile, and speedometer error — 255/35R20 is the taller tire, with a 3.9% speedo delta when swapping.

255/30R20

26.02in

661 mm overall diameter

255/35R20

27.03in

687 mm overall diameter

Diameter Δ

1 in

Speedo error

3.9%

255/30R20 revs/mi

775

255/35R20 revs/mi

746

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	255/30R20	255/35R20
Section width	255 mm	255 mm
Aspect ratio	30	35
Wheel diameter	20″	20″
Sidewall height	77 mm	89 mm
Overall diameter	26.02 in (661 mm)	27.03 in (687 mm)
Circumference	81.76 in	84.91 in
Revolutions / mile	775	746

Other comparisons for these sizes

235/40R18 vs 255/30R20 235/45R18 vs 255/35R20 245/35R19 vs 255/30R20 245/40R19 vs 255/35R20 255/35R19 vs 255/30R20 255/40R19 vs 255/35R20 245/35R20 vs 255/35R20 245/35R20 vs 255/30R20 255/30R20 vs 265/30R20 255/30R20 vs 275/30R20

FAQ

Frequently asked

What is the diameter difference between 255/30R20 and 255/35R20?: 255/30R20 has an overall diameter of 26.02 inches (661 mm). 255/35R20 is 27.03 inches (687 mm). That's a 1 inch (26 mm) difference — 255/35R20 is taller.
Will 255/35R20 cause speedometer error on a car fitted with 255/30R20?: Swapping 255/30R20 for 255/35R20 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/30R20 turns 775 revs per mile. 255/35R20 turns 746 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/30R20 has a sidewall of 77 mm (3.01 in). 255/35R20 sidewall is 89 mm (3.51 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/35R20 a safe plus-size upgrade from 255/30R20?: 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.