What is the diameter difference between 245/40R18 and 265/30R20?

245/40R18 has an overall diameter of 25.72 inches (653 mm). 265/30R20 is 26.26 inches (667 mm). That's a 0.54 inch (14 mm) difference — 265/30R20 is taller.

Will 265/30R20 cause speedometer error on a car fitted with 245/40R18?

Swapping 245/40R18 for 265/30R20 produces a speedometer error of about 2.1%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.3 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. 265/30R20 turns 768 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). 265/30R20 sidewall is 80 mm (3.13 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 265/30R20 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 2.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

245/40R18 vs 265/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 265/30R20 is the taller tire, with a 2.1% speedo delta when swapping.

245/40R18

25.72in

653 mm overall diameter

265/30R20

26.26in

667 mm overall diameter

Diameter Δ

0.54 in

Speedo error

2.1%

245/40R18 revs/mi

784

265/30R20 revs/mi

768

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	245/40R18	265/30R20
Section width	245 mm	265 mm
Aspect ratio	40	30
Wheel diameter	18″	20″
Sidewall height	98 mm	80 mm
Overall diameter	25.72 in (653 mm)	26.26 in (667 mm)
Circumference	80.79 in	82.5 in
Revolutions / mile	784	768

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 245/40R18 and 265/30R20?: 245/40R18 has an overall diameter of 25.72 inches (653 mm). 265/30R20 is 26.26 inches (667 mm). That's a 0.54 inch (14 mm) difference — 265/30R20 is taller.
Will 265/30R20 cause speedometer error on a car fitted with 245/40R18?: Swapping 245/40R18 for 265/30R20 produces a speedometer error of about 2.1%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.3 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. 265/30R20 turns 768 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). 265/30R20 sidewall is 80 mm (3.13 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 265/30R20 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 2.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.