What is the diameter difference between 235/45R17 and 235/40R18?

235/45R17 has an overall diameter of 25.33 inches (643 mm). 235/40R18 is 25.4 inches (645 mm). That's a 0.07 inch (2 mm) difference — 235/40R18 is taller.

Will 235/40R18 cause speedometer error on a car fitted with 235/45R17?

Swapping 235/45R17 for 235/40R18 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?

235/45R17 turns 796 revs per mile. 235/40R18 turns 794 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

235/45R17 has a sidewall of 106 mm (4.16 in). 235/40R18 sidewall is 94 mm (3.7 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 235/40R18 a safe plus-size upgrade from 235/45R17?

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

235/45R17 vs 235/40R18

Overall diameter, sidewall, revs per mile, and speedometer error — 235/40R18 is the taller tire, with a 0.3% speedo delta when swapping.

235/45R17

25.33in

643 mm overall diameter

235/40R18

25.4in

645 mm overall diameter

Diameter Δ

0.07 in

Speedo error

0.3%

235/45R17 revs/mi

796

235/40R18 revs/mi

794

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	235/45R17	235/40R18
Section width	235 mm	235 mm
Aspect ratio	45	40
Wheel diameter	17″	18″
Sidewall height	106 mm	94 mm
Overall diameter	25.33 in (643 mm)	25.4 in (645 mm)
Circumference	79.57 in	79.8 in
Revolutions / mile	796	794

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 235/45R17 and 235/40R18?: 235/45R17 has an overall diameter of 25.33 inches (643 mm). 235/40R18 is 25.4 inches (645 mm). That's a 0.07 inch (2 mm) difference — 235/40R18 is taller.
Will 235/40R18 cause speedometer error on a car fitted with 235/45R17?: Swapping 235/45R17 for 235/40R18 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?: 235/45R17 turns 796 revs per mile. 235/40R18 turns 794 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 235/45R17 has a sidewall of 106 mm (4.16 in). 235/40R18 sidewall is 94 mm (3.7 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 235/40R18 a safe plus-size upgrade from 235/45R17?: 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.