What is the diameter difference between 255/40R18 and 255/45R18?

255/40R18 has an overall diameter of 26.03 inches (661 mm). 255/45R18 is 27.04 inches (687 mm). That's a 1 inch (26 mm) difference — 255/45R18 is taller.

Will 255/45R18 cause speedometer error on a car fitted with 255/40R18?

Swapping 255/40R18 for 255/45R18 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 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. 255/45R18 turns 746 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). 255/45R18 sidewall is 115 mm (4.52 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/45R18 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 3.9%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

255/40R18 vs 255/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 255/45R18 is the taller tire, with a 3.9% speedo delta when swapping.

255/40R18

26.03in

661 mm overall diameter

255/45R18

27.04in

687 mm overall diameter

Diameter Δ

1 in

Speedo error

3.9%

255/40R18 revs/mi

775

255/45R18 revs/mi

746

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	255/40R18	255/45R18
Section width	255 mm	255 mm
Aspect ratio	40	45
Wheel diameter	18″	18″
Sidewall height	102 mm	115 mm
Overall diameter	26.03 in (661 mm)	27.04 in (687 mm)
Circumference	81.78 in	84.93 in
Revolutions / mile	775	746

Other comparisons for these sizes

245/45R17 vs 255/40R18 235/40R18 vs 255/40R18 235/45R18 vs 255/45R18 245/40R18 vs 255/40R18 245/45R18 vs 255/45R18 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

FAQ

Frequently asked

What is the diameter difference between 255/40R18 and 255/45R18?: 255/40R18 has an overall diameter of 26.03 inches (661 mm). 255/45R18 is 27.04 inches (687 mm). That's a 1 inch (26 mm) difference — 255/45R18 is taller.
Will 255/45R18 cause speedometer error on a car fitted with 255/40R18?: Swapping 255/40R18 for 255/45R18 produces a speedometer error of about 3.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 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. 255/45R18 turns 746 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). 255/45R18 sidewall is 115 mm (4.52 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/45R18 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 3.9%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.