What is the diameter difference between 255/40R18 and 275/40R18?

255/40R18 has an overall diameter of 26.03 inches (661 mm). 275/40R18 is 26.66 inches (677 mm). That's a 0.63 inch (16 mm) difference — 275/40R18 is taller.

Will 275/40R18 cause speedometer error on a car fitted with 255/40R18?

Swapping 255/40R18 for 275/40R18 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?

255/40R18 turns 775 revs per mile. 275/40R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/40R18 has a sidewall of 102 mm (4.02 in). 275/40R18 sidewall is 110 mm (4.33 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/40R18 a safe plus-size upgrade from 255/40R18?

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

255/40R18 vs 275/40R18

Overall diameter, sidewall, revs per mile, and speedometer error — 275/40R18 is the taller tire, with a 2.4% speedo delta when swapping.

255/40R18

26.03in

661 mm overall diameter

275/40R18

26.66in

677 mm overall diameter

Diameter Δ

0.63 in

Speedo error

2.4%

255/40R18 revs/mi

775

275/40R18 revs/mi

756

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	255/40R18	275/40R18
Section width	255 mm	275 mm
Aspect ratio	40	40
Wheel diameter	18″	18″
Sidewall height	102 mm	110 mm
Overall diameter	26.03 in (661 mm)	26.66 in (677 mm)
Circumference	81.78 in	83.76 in
Revolutions / mile	775	756

Other comparisons for these sizes

245/45R17 vs 255/40R18 235/40R18 vs 255/40R18 245/40R18 vs 255/40R18 245/45R18 vs 255/40R18 255/35R18 vs 255/40R18 255/40R18 vs 265/35R19 255/40R18 vs 255/35R19 255/40R18 vs 275/30R20 255/40R18 vs 255/45R18 255/40R18 vs 265/35R18

FAQ

Frequently asked

What is the diameter difference between 255/40R18 and 275/40R18?: 255/40R18 has an overall diameter of 26.03 inches (661 mm). 275/40R18 is 26.66 inches (677 mm). That's a 0.63 inch (16 mm) difference — 275/40R18 is taller.
Will 275/40R18 cause speedometer error on a car fitted with 255/40R18?: Swapping 255/40R18 for 275/40R18 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?: 255/40R18 turns 775 revs per mile. 275/40R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/40R18 has a sidewall of 102 mm (4.02 in). 275/40R18 sidewall is 110 mm (4.33 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/40R18 a safe plus-size upgrade from 255/40R18?: 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.