What is the diameter difference between 225/50R17 and 235/45R18?

225/50R17 has an overall diameter of 25.86 inches (657 mm). 235/45R18 is 26.33 inches (669 mm). That's a 0.47 inch (12 mm) difference — 235/45R18 is taller.

Will 235/45R18 cause speedometer error on a car fitted with 225/50R17?

Swapping 225/50R17 for 235/45R18 produces a speedometer error of about 1.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

225/50R17 turns 780 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?

225/50R17 has a sidewall of 113 mm (4.43 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 225/50R17?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

225/50R17 vs 235/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/45R18 is the taller tire, with a 1.8% speedo delta when swapping.

225/50R17

25.86in

657 mm overall diameter

235/45R18

26.33in

669 mm overall diameter

Diameter Δ

0.47 in

Speedo error

1.8%

225/50R17 revs/mi

780

235/45R18 revs/mi

766

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	225/50R17	235/45R18
Section width	225 mm	235 mm
Aspect ratio	50	45
Wheel diameter	17″	18″
Sidewall height	113 mm	106 mm
Overall diameter	25.86 in (657 mm)	26.33 in (669 mm)
Circumference	81.24 in	82.71 in
Revolutions / mile	780	766

Other comparisons for these sizes

205/50R17 vs 225/50R17 215/55R17 vs 225/50R17 215/50R17 vs 225/50R17 225/50R17 vs 225/45R18 225/50R17 vs 245/40R19 225/50R17 vs 225/45R17 225/50R17 vs 235/45R17 225/45R18 vs 235/45R18 235/40R18 vs 235/45R18 235/45R18 vs 245/40R19

FAQ

Frequently asked

What is the diameter difference between 225/50R17 and 235/45R18?: 225/50R17 has an overall diameter of 25.86 inches (657 mm). 235/45R18 is 26.33 inches (669 mm). That's a 0.47 inch (12 mm) difference — 235/45R18 is taller.
Will 235/45R18 cause speedometer error on a car fitted with 225/50R17?: Swapping 225/50R17 for 235/45R18 produces a speedometer error of about 1.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 225/50R17 turns 780 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?: 225/50R17 has a sidewall of 113 mm (4.43 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 225/50R17?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.