What is the diameter difference between 255/35R18 and 255/40R18?

255/35R18 has an overall diameter of 25.03 inches (636 mm). 255/40R18 is 26.03 inches (661 mm). That's a 1 inch (26 mm) difference — 255/40R18 is taller.

Will 255/40R18 cause speedometer error on a car fitted with 255/35R18?

Swapping 255/35R18 for 255/40R18 produces a speedometer error of about 4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.4 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/35R18 turns 806 revs per mile. 255/40R18 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/35R18 has a sidewall of 89 mm (3.51 in). 255/40R18 sidewall is 102 mm (4.02 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/40R18 a safe plus-size upgrade from 255/35R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

255/35R18 vs 255/40R18

Overall diameter, sidewall, revs per mile, and speedometer error — 255/40R18 is the taller tire, with a 4% speedo delta when swapping.

255/35R18

25.03in

636 mm overall diameter

255/40R18

26.03in

661 mm overall diameter

Diameter Δ

1 in

Speedo error

255/35R18 revs/mi

806

255/40R18 revs/mi

775

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	255/35R18	255/40R18
Section width	255 mm	255 mm
Aspect ratio	35	40
Wheel diameter	18″	18″
Sidewall height	89 mm	102 mm
Overall diameter	25.03 in (636 mm)	26.03 in (661 mm)
Circumference	78.63 in	81.78 in
Revolutions / mile	806	775

Other comparisons for these sizes

245/45R17 vs 255/40R18 235/40R18 vs 255/40R18 245/40R18 vs 255/40R18 245/40R18 vs 255/35R18 245/45R18 vs 255/40R18 255/35R18 vs 265/30R19 255/35R18 vs 265/35R18 255/35R18 vs 275/35R18 255/40R18 vs 265/35R19 255/40R18 vs 255/35R19

FAQ

Frequently asked

What is the diameter difference between 255/35R18 and 255/40R18?: 255/35R18 has an overall diameter of 25.03 inches (636 mm). 255/40R18 is 26.03 inches (661 mm). That's a 1 inch (26 mm) difference — 255/40R18 is taller.
Will 255/40R18 cause speedometer error on a car fitted with 255/35R18?: Swapping 255/35R18 for 255/40R18 produces a speedometer error of about 4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.4 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/35R18 turns 806 revs per mile. 255/40R18 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/35R18 has a sidewall of 89 mm (3.51 in). 255/40R18 sidewall is 102 mm (4.02 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/40R18 a safe plus-size upgrade from 255/35R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.