What is the diameter difference between 245/45R17 and 255/40R18?

245/45R17 has an overall diameter of 25.68 inches (652 mm). 255/40R18 is 26.03 inches (661 mm). That's a 0.35 inch (9 mm) difference — 255/40R18 is taller.

Will 255/40R18 cause speedometer error on a car fitted with 245/45R17?

Swapping 245/45R17 for 255/40R18 produces a speedometer error of about 1.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.8 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

245/45R17 turns 785 revs per mile. 255/40R18 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

245/45R17 has a sidewall of 110 mm (4.34 in). 255/40R18 sidewall is 102 mm (4.02 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 255/40R18 a safe plus-size upgrade from 245/45R17?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

245/45R17 vs 255/40R18

Overall diameter, sidewall, revs per mile, and speedometer error — 255/40R18 is the taller tire, with a 1.4% speedo delta when swapping.

245/45R17

25.68in

652 mm overall diameter

255/40R18

26.03in

661 mm overall diameter

Diameter Δ

0.35 in

Speedo error

1.4%

245/45R17 revs/mi

785

255/40R18 revs/mi

775

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	245/45R17	255/40R18
Section width	245 mm	255 mm
Aspect ratio	45	40
Wheel diameter	17″	18″
Sidewall height	110 mm	102 mm
Overall diameter	25.68 in (652 mm)	26.03 in (661 mm)
Circumference	80.68 in	81.78 in
Revolutions / mile	785	775

Other comparisons for these sizes

225/45R17 vs 245/45R17 235/45R17 vs 245/45R17 245/45R17 vs 245/40R18 245/45R17 vs 265/35R19 235/40R18 vs 255/40R18 245/40R18 vs 255/40R18 245/45R18 vs 255/40R18 255/35R18 vs 255/40R18 255/40R18 vs 265/35R19 255/40R18 vs 255/35R19

FAQ

Frequently asked

What is the diameter difference between 245/45R17 and 255/40R18?: 245/45R17 has an overall diameter of 25.68 inches (652 mm). 255/40R18 is 26.03 inches (661 mm). That's a 0.35 inch (9 mm) difference — 255/40R18 is taller.
Will 255/40R18 cause speedometer error on a car fitted with 245/45R17?: Swapping 245/45R17 for 255/40R18 produces a speedometer error of about 1.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.8 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 245/45R17 turns 785 revs per mile. 255/40R18 turns 775 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 245/45R17 has a sidewall of 110 mm (4.34 in). 255/40R18 sidewall is 102 mm (4.02 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 255/40R18 a safe plus-size upgrade from 245/45R17?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.4%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.