What is the diameter difference between 235/40R18 and 255/30R20?

235/40R18 has an overall diameter of 25.4 inches (645 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0.62 inch (16 mm) difference — 255/30R20 is taller.

Will 255/30R20 cause speedometer error on a car fitted with 235/40R18?

Swapping 235/40R18 for 255/30R20 produces a speedometer error of about 2.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.5 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

235/40R18 turns 794 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?

235/40R18 has a sidewall of 94 mm (3.7 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 235/40R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

235/40R18 vs 255/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 255/30R20 is the taller tire, with a 2.4% speedo delta when swapping.

235/40R18

25.4in

645 mm overall diameter

255/30R20

26.02in

661 mm overall diameter

Diameter Δ

0.62 in

Speedo error

2.4%

235/40R18 revs/mi

794

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	235/40R18	255/30R20
Section width	235 mm	255 mm
Aspect ratio	40	30
Wheel diameter	18″	20″
Sidewall height	94 mm	77 mm
Overall diameter	25.4 in (645 mm)	26.02 in (661 mm)
Circumference	79.8 in	81.76 in
Revolutions / mile	794	775

Other comparisons for these sizes

225/45R17 vs 235/40R18 235/45R17 vs 235/40R18 215/40R18 vs 235/40R18 225/40R18 vs 235/40R18 225/45R18 vs 235/40R18 235/40R18 vs 245/35R19 235/40R18 vs 235/35R19 235/40R18 vs 245/40R18 235/40R18 vs 255/40R18 235/40R18 vs 235/45R18

FAQ

Frequently asked

What is the diameter difference between 235/40R18 and 255/30R20?: 235/40R18 has an overall diameter of 25.4 inches (645 mm). 255/30R20 is 26.02 inches (661 mm). That's a 0.62 inch (16 mm) difference — 255/30R20 is taller.
Will 255/30R20 cause speedometer error on a car fitted with 235/40R18?: Swapping 235/40R18 for 255/30R20 produces a speedometer error of about 2.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.5 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 235/40R18 turns 794 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?: 235/40R18 has a sidewall of 94 mm (3.7 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 235/40R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.