What is the diameter difference between 245/40R18 and 245/45R18?

245/40R18 has an overall diameter of 25.72 inches (653 mm). 245/45R18 is 26.68 inches (678 mm). That's a 0.96 inch (25 mm) difference — 245/45R18 is taller.

Will 245/45R18 cause speedometer error on a car fitted with 245/40R18?

Swapping 245/40R18 for 245/45R18 produces a speedometer error of about 3.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

245/40R18 turns 784 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?

245/40R18 has a sidewall of 98 mm (3.86 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 245/40R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.8%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

245/40R18 vs 245/45R18

Overall diameter, sidewall, revs per mile, and speedometer error — 245/45R18 is the taller tire, with a 3.8% speedo delta when swapping.

245/40R18

25.72in

653 mm overall diameter

245/45R18

26.68in

678 mm overall diameter

Diameter Δ

0.96 in

Speedo error

3.8%

245/40R18 revs/mi

784

245/45R18 revs/mi

756

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	245/40R18	245/45R18
Section width	245 mm	245 mm
Aspect ratio	40	45
Wheel diameter	18″	18″
Sidewall height	98 mm	110 mm
Overall diameter	25.72 in (653 mm)	26.68 in (678 mm)
Circumference	80.79 in	83.82 in
Revolutions / mile	784	756

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 245/40R18 and 245/45R18?: 245/40R18 has an overall diameter of 25.72 inches (653 mm). 245/45R18 is 26.68 inches (678 mm). That's a 0.96 inch (25 mm) difference — 245/45R18 is taller.
Will 245/45R18 cause speedometer error on a car fitted with 245/40R18?: Swapping 245/40R18 for 245/45R18 produces a speedometer error of about 3.8%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.3 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 245/40R18 turns 784 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?: 245/40R18 has a sidewall of 98 mm (3.86 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 245/40R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.8%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.