What is the diameter difference between 245/35R20 and 255/30R20?

245/35R20 has an overall diameter of 26.75 inches (680 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0.73 inch (19 mm) difference — 255/30R20 is shorter.

Will 255/30R20 cause speedometer error on a car fitted with 245/35R20?

Swapping 245/35R20 for 255/30R20 produces a speedometer error of about 2.7%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.4 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

245/35R20 turns 754 revs per mile. 255/30R20 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

245/35R20 has a sidewall of 86 mm (3.38 in). 255/30R20 sidewall is 77 mm (3.01 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/30R20 a safe plus-size upgrade from 245/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

245/35R20 vs 255/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 245/35R20 is the taller tire, with a 2.7% speedo delta when swapping.

245/35R20

26.75in

680 mm overall diameter

255/30R20

26.02in

661 mm overall diameter

Diameter Δ

0.73 in

Speedo error

2.7%

245/35R20 revs/mi

754

255/30R20 revs/mi

775

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	245/35R20	255/30R20
Section width	245 mm	255 mm
Aspect ratio	35	30
Wheel diameter	20″	20″
Sidewall height	86 mm	77 mm
Overall diameter	26.75 in (680 mm)	26.02 in (661 mm)
Circumference	84.04 in	81.76 in
Revolutions / mile	754	775

Other comparisons for these sizes

225/45R18 vs 245/35R20 235/40R18 vs 255/30R20 245/35R19 vs 255/30R20 245/40R19 vs 245/35R20 255/35R19 vs 255/30R20 245/35R20 vs 255/35R20 245/35R20 vs 265/35R20 255/30R20 vs 265/30R20 255/30R20 vs 275/30R20 255/30R20 vs 255/35R20

FAQ

Frequently asked

What is the diameter difference between 245/35R20 and 255/30R20?: 245/35R20 has an overall diameter of 26.75 inches (680 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0.73 inch (19 mm) difference — 255/30R20 is shorter.
Will 255/30R20 cause speedometer error on a car fitted with 245/35R20?: Swapping 245/35R20 for 255/30R20 produces a speedometer error of about 2.7%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.4 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 245/35R20 turns 754 revs per mile. 255/30R20 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 245/35R20 has a sidewall of 86 mm (3.38 in). 255/30R20 sidewall is 77 mm (3.01 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/30R20 a safe plus-size upgrade from 245/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.