What is the diameter difference between 235/50R18 and 245/45R18?

235/50R18 has an overall diameter of 27.25 inches (692 mm). 245/45R18 is 26.68 inches (678 mm). That's a 0.57 inch (15 mm) difference — 245/45R18 is shorter.

Will 245/45R18 cause speedometer error on a car fitted with 235/50R18?

Swapping 235/50R18 for 245/45R18 produces a speedometer error of about 2.1%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.7 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

235/50R18 turns 740 revs per mile. 245/45R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

235/50R18 has a sidewall of 118 mm (4.63 in). 245/45R18 sidewall is 110 mm (4.34 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 245/45R18 a safe plus-size upgrade from 235/50R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

235/50R18 vs 245/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/50R18 is the taller tire, with a 2.1% speedo delta when swapping.

235/50R18

27.25in

692 mm overall diameter

245/45R18

26.68in

678 mm overall diameter

Diameter Δ

0.57 in

Speedo error

2.1%

235/50R18 revs/mi

740

245/45R18 revs/mi

756

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	235/50R18	245/45R18
Section width	235 mm	245 mm
Aspect ratio	50	45
Wheel diameter	18″	18″
Sidewall height	118 mm	110 mm
Overall diameter	27.25 in (692 mm)	26.68 in (678 mm)
Circumference	85.61 in	83.82 in
Revolutions / mile	740	756

Other comparisons for these sizes

215/60R16 vs 235/50R18 235/55R17 vs 235/50R18 225/45R18 vs 245/45R18 235/45R18 vs 245/45R18 235/45R18 vs 235/50R18 235/50R18 vs 245/45R19 245/40R18 vs 245/45R18 245/45R18 vs 255/40R19 245/45R18 vs 245/40R19 245/45R18 vs 265/35R20

FAQ

Frequently asked

What is the diameter difference between 235/50R18 and 245/45R18?: 235/50R18 has an overall diameter of 27.25 inches (692 mm). 245/45R18 is 26.68 inches (678 mm). That's a 0.57 inch (15 mm) difference — 245/45R18 is shorter.
Will 245/45R18 cause speedometer error on a car fitted with 235/50R18?: Swapping 235/50R18 for 245/45R18 produces a speedometer error of about 2.1%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.7 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 235/50R18 turns 740 revs per mile. 245/45R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 235/50R18 has a sidewall of 118 mm (4.63 in). 245/45R18 sidewall is 110 mm (4.34 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 245/45R18 a safe plus-size upgrade from 235/50R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.