What is the diameter difference between 225/40R18 and 225/45R18?

225/40R18 has an overall diameter of 25.09 inches (637 mm). 225/45R18 is 25.97 inches (660 mm). That's a 0.89 inch (23 mm) difference — 225/45R18 is taller.

Will 225/45R18 cause speedometer error on a car fitted with 225/40R18?

Swapping 225/40R18 for 225/45R18 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?

225/40R18 turns 804 revs per mile. 225/45R18 turns 777 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

225/40R18 has a sidewall of 90 mm (3.54 in). 225/45R18 sidewall is 101 mm (3.99 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 225/45R18 a safe plus-size upgrade from 225/40R18?

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

225/40R18 vs 225/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 225/45R18 is the taller tire, with a 3.5% speedo delta when swapping.

225/40R18

25.09in

637 mm overall diameter

225/45R18

25.97in

660 mm overall diameter

Diameter Δ

0.89 in

Speedo error

3.5%

225/40R18 revs/mi

804

225/45R18 revs/mi

777

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	225/40R18	225/45R18
Section width	225 mm	225 mm
Aspect ratio	40	45
Wheel diameter	18″	18″
Sidewall height	90 mm	101 mm
Overall diameter	25.09 in (637 mm)	25.97 in (660 mm)
Circumference	78.81 in	81.59 in
Revolutions / mile	804	777

Other comparisons for these sizes

205/55R16 vs 225/45R18 215/50R17 vs 225/45R18 225/50R17 vs 225/45R18 225/45R17 vs 225/40R18 215/40R18 vs 225/40R18 225/40R18 vs 235/35R19 225/40R18 vs 225/35R19 225/40R18 vs 235/40R18 225/40R18 vs 245/40R18 225/45R18 vs 225/40R19

FAQ

Frequently asked

What is the diameter difference between 225/40R18 and 225/45R18?: 225/40R18 has an overall diameter of 25.09 inches (637 mm). 225/45R18 is 25.97 inches (660 mm). That's a 0.89 inch (23 mm) difference — 225/45R18 is taller.
Will 225/45R18 cause speedometer error on a car fitted with 225/40R18?: Swapping 225/40R18 for 225/45R18 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?: 225/40R18 turns 804 revs per mile. 225/45R18 turns 777 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 225/40R18 has a sidewall of 90 mm (3.54 in). 225/45R18 sidewall is 101 mm (3.99 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 225/45R18 a safe plus-size upgrade from 225/40R18?: 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.