What is the diameter difference between 235/40R18 and 235/45R18?

235/40R18 has an overall diameter of 25.4 inches (645 mm). 235/45R18 is 26.33 inches (669 mm). That's a 0.93 inch (24 mm) difference — 235/45R18 is taller.

Will 235/45R18 cause speedometer error on a car fitted with 235/40R18?

Swapping 235/40R18 for 235/45R18 produces a speedometer error of about 3.6%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.2 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. 235/45R18 turns 766 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). 235/45R18 sidewall is 106 mm (4.16 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 235/45R18 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 3.6%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

235/40R18 vs 235/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/45R18 is the taller tire, with a 3.6% speedo delta when swapping.

235/40R18

25.4in

645 mm overall diameter

235/45R18

26.33in

669 mm overall diameter

Diameter Δ

0.93 in

Speedo error

3.6%

235/40R18 revs/mi

794

235/45R18 revs/mi

766

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	235/40R18	235/45R18
Section width	235 mm	235 mm
Aspect ratio	40	45
Wheel diameter	18″	18″
Sidewall height	94 mm	106 mm
Overall diameter	25.4 in (645 mm)	26.33 in (669 mm)
Circumference	79.8 in	82.71 in
Revolutions / mile	794	766

Other comparisons for these sizes

225/50R17 vs 235/45R18 225/45R17 vs 235/40R18 235/45R17 vs 235/40R18 215/40R18 vs 235/40R18 225/40R18 vs 235/40R18 225/45R18 vs 235/45R18 225/45R18 vs 235/40R18 235/40R18 vs 245/35R19 235/40R18 vs 235/35R19 235/40R18 vs 255/30R20

FAQ

Frequently asked

What is the diameter difference between 235/40R18 and 235/45R18?: 235/40R18 has an overall diameter of 25.4 inches (645 mm). 235/45R18 is 26.33 inches (669 mm). That's a 0.93 inch (24 mm) difference — 235/45R18 is taller.
Will 235/45R18 cause speedometer error on a car fitted with 235/40R18?: Swapping 235/40R18 for 235/45R18 produces a speedometer error of about 3.6%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.2 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. 235/45R18 turns 766 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). 235/45R18 sidewall is 106 mm (4.16 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 235/45R18 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 3.6%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.