What is the diameter difference between 235/55R17 and 235/50R18?

235/55R17 has an overall diameter of 27.18 inches (690 mm). 235/50R18 is 27.25 inches (692 mm). That's a 0.07 inch (2 mm) difference — 235/50R18 is taller.

Will 235/50R18 cause speedometer error on a car fitted with 235/55R17?

Swapping 235/55R17 for 235/50R18 produces a speedometer error of about 0.3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.2 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

235/55R17 turns 742 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?

235/55R17 has a sidewall of 129 mm (5.09 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 235/55R17?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

235/55R17 vs 235/50R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/50R18 is the taller tire, with a 0.3% speedo delta when swapping.

235/55R17

27.18in

690 mm overall diameter

235/50R18

27.25in

692 mm overall diameter

Diameter Δ

0.07 in

Speedo error

0.3%

235/55R17 revs/mi

742

235/50R18 revs/mi

740

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	235/55R17	235/50R18
Section width	235 mm	235 mm
Aspect ratio	55	50
Wheel diameter	17″	18″
Sidewall height	129 mm	118 mm
Overall diameter	27.18 in (690 mm)	27.25 in (692 mm)
Circumference	85.38 in	85.61 in
Revolutions / mile	742	740

Other comparisons for these sizes

215/60R16 vs 235/50R18 225/60R16 vs 235/55R17 215/55R17 vs 235/55R17 235/45R18 vs 235/50R18 235/50R18 vs 245/45R19 235/50R18 vs 245/45R18

FAQ

Frequently asked

What is the diameter difference between 235/55R17 and 235/50R18?: 235/55R17 has an overall diameter of 27.18 inches (690 mm). 235/50R18 is 27.25 inches (692 mm). That's a 0.07 inch (2 mm) difference — 235/50R18 is taller.
Will 235/50R18 cause speedometer error on a car fitted with 235/55R17?: Swapping 235/55R17 for 235/50R18 produces a speedometer error of about 0.3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.2 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 235/55R17 turns 742 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?: 235/55R17 has a sidewall of 129 mm (5.09 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 235/55R17?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.