What is the diameter difference between 255/35R19 and 255/30R20?

255/35R19 has an overall diameter of 26.03 inches (661 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0 inch (0 mm) difference — 255/30R20 is shorter.

Will 255/30R20 cause speedometer error on a car fitted with 255/35R19?

Swapping 255/35R19 for 255/30R20 produces a speedometer error of about 0%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 60 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/35R19 turns 775 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?

255/35R19 has a sidewall of 89 mm (3.51 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 255/35R19?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

255/35R19 vs 255/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 255/35R19 is the taller tire, with a 0% speedo delta when swapping.

255/35R19

26.03in

661 mm overall diameter

255/30R20

26.02in

661 mm overall diameter

Diameter Δ

0 in

Speedo error

255/35R19 revs/mi

775

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	255/35R19	255/30R20
Section width	255 mm	255 mm
Aspect ratio	35	30
Wheel diameter	19″	20″
Sidewall height	89 mm	77 mm
Overall diameter	26.03 in (661 mm)	26.02 in (661 mm)
Circumference	81.77 in	81.76 in
Revolutions / mile	775	775

Other comparisons for these sizes

235/45R17 vs 255/35R19 235/40R18 vs 255/30R20 245/40R18 vs 255/35R19 255/40R18 vs 255/35R19 235/35R19 vs 255/35R19 245/35R19 vs 255/30R20 245/35R19 vs 255/35R19 245/40R19 vs 255/35R19 255/35R19 vs 265/30R20 255/35R19 vs 265/35R19

FAQ

Frequently asked

What is the diameter difference between 255/35R19 and 255/30R20?: 255/35R19 has an overall diameter of 26.03 inches (661 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0 inch (0 mm) difference — 255/30R20 is shorter.
Will 255/30R20 cause speedometer error on a car fitted with 255/35R19?: Swapping 255/35R19 for 255/30R20 produces a speedometer error of about 0%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 60 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/35R19 turns 775 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?: 255/35R19 has a sidewall of 89 mm (3.51 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 255/35R19?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.