What is the diameter difference between 245/40R18 and 255/35R18?

245/40R18 has an overall diameter of 25.72 inches (653 mm). 255/35R18 is 25.03 inches (636 mm). That's a 0.69 inch (18 mm) difference — 255/35R18 is shorter.

Will 255/35R18 cause speedometer error on a car fitted with 245/40R18?

Swapping 245/40R18 for 255/35R18 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?

245/40R18 turns 784 revs per mile. 255/35R18 turns 806 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

245/40R18 has a sidewall of 98 mm (3.86 in). 255/35R18 sidewall is 89 mm (3.51 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/35R18 a safe plus-size upgrade from 245/40R18?

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

245/40R18 vs 255/35R18

Overall diameter, sidewall, revs per mile, and speedometer error — 245/40R18 is the taller tire, with a 2.7% speedo delta when swapping.

245/40R18

25.72in

653 mm overall diameter

255/35R18

25.03in

636 mm overall diameter

Diameter Δ

0.69 in

Speedo error

2.7%

245/40R18 revs/mi

784

255/35R18 revs/mi

806

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	245/40R18	255/35R18
Section width	245 mm	255 mm
Aspect ratio	40	35
Wheel diameter	18″	18″
Sidewall height	98 mm	89 mm
Overall diameter	25.72 in (653 mm)	25.03 in (636 mm)
Circumference	80.79 in	78.63 in
Revolutions / mile	784	806

Other comparisons for these sizes

235/45R17 vs 245/40R18 245/45R17 vs 245/40R18 225/40R18 vs 245/40R18 235/40R18 vs 245/40R18 235/45R18 vs 245/40R18 245/40R18 vs 255/35R19 245/40R18 vs 245/35R19 245/40R18 vs 265/30R20 245/40R18 vs 255/40R18 245/40R18 vs 245/45R18

FAQ

Frequently asked

What is the diameter difference between 245/40R18 and 255/35R18?: 245/40R18 has an overall diameter of 25.72 inches (653 mm). 255/35R18 is 25.03 inches (636 mm). That's a 0.69 inch (18 mm) difference — 255/35R18 is shorter.
Will 255/35R18 cause speedometer error on a car fitted with 245/40R18?: Swapping 245/40R18 for 255/35R18 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?: 245/40R18 turns 784 revs per mile. 255/35R18 turns 806 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 245/40R18 has a sidewall of 98 mm (3.86 in). 255/35R18 sidewall is 89 mm (3.51 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/35R18 a safe plus-size upgrade from 245/40R18?: 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.