What is the diameter difference between 235/45R18 and 235/50R18?

235/45R18 has an overall diameter of 26.33 inches (669 mm). 235/50R18 is 27.25 inches (692 mm). That's a 0.93 inch (24 mm) difference — 235/50R18 is taller.

Will 235/50R18 cause speedometer error on a car fitted with 235/45R18?

Swapping 235/45R18 for 235/50R18 produces a speedometer error of about 3.5%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

235/45R18 turns 766 revs per mile. 235/50R18 turns 740 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

235/45R18 has a sidewall of 106 mm (4.16 in). 235/50R18 sidewall is 118 mm (4.63 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 235/50R18 a safe plus-size upgrade from 235/45R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.5%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

235/45R18 vs 235/50R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/50R18 is the taller tire, with a 3.5% speedo delta when swapping.

235/45R18

26.33in

669 mm overall diameter

235/50R18

27.25in

692 mm overall diameter

Diameter Δ

0.93 in

Speedo error

3.5%

235/45R18 revs/mi

766

235/50R18 revs/mi

740

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	235/45R18	235/50R18
Section width	235 mm	235 mm
Aspect ratio	45	50
Wheel diameter	18″	18″
Sidewall height	106 mm	118 mm
Overall diameter	26.33 in (669 mm)	27.25 in (692 mm)
Circumference	82.71 in	85.61 in
Revolutions / mile	766	740

Other comparisons for these sizes

215/60R16 vs 235/50R18 225/50R17 vs 235/45R18 235/55R17 vs 235/50R18 225/45R18 vs 235/45R18 235/40R18 vs 235/45R18 235/45R18 vs 245/40R19 235/45R18 vs 255/35R20 235/45R18 vs 245/45R18 235/45R18 vs 255/45R18 235/45R18 vs 245/40R18

FAQ

Frequently asked

What is the diameter difference between 235/45R18 and 235/50R18?: 235/45R18 has an overall diameter of 26.33 inches (669 mm). 235/50R18 is 27.25 inches (692 mm). That's a 0.93 inch (24 mm) difference — 235/50R18 is taller.
Will 235/50R18 cause speedometer error on a car fitted with 235/45R18?: Swapping 235/45R18 for 235/50R18 produces a speedometer error of about 3.5%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 235/45R18 turns 766 revs per mile. 235/50R18 turns 740 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 235/45R18 has a sidewall of 106 mm (4.16 in). 235/50R18 sidewall is 118 mm (4.63 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 235/50R18 a safe plus-size upgrade from 235/45R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.5%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.