What is the diameter difference between 275/35R18 and 275/40R18?

275/35R18 has an overall diameter of 25.58 inches (650 mm). 275/40R18 is 26.66 inches (677 mm). That's a 1.08 inch (28 mm) difference — 275/40R18 is taller.

Will 275/40R18 cause speedometer error on a car fitted with 275/35R18?

Swapping 275/35R18 for 275/40R18 produces a speedometer error of about 4.2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.5 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

275/35R18 turns 788 revs per mile. 275/40R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

275/35R18 has a sidewall of 96 mm (3.79 in). 275/40R18 sidewall is 110 mm (4.33 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/40R18 a safe plus-size upgrade from 275/35R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4.2%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

275/35R18 vs 275/40R18

Overall diameter, sidewall, revs per mile, and speedometer error — 275/40R18 is the taller tire, with a 4.2% speedo delta when swapping.

275/35R18

25.58in

650 mm overall diameter

275/40R18

26.66in

677 mm overall diameter

Diameter Δ

1.08 in

Speedo error

4.2%

275/35R18 revs/mi

788

275/40R18 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	275/35R18	275/40R18
Section width	275 mm	275 mm
Aspect ratio	35	40
Wheel diameter	18″	18″
Sidewall height	96 mm	110 mm
Overall diameter	25.58 in (650 mm)	26.66 in (677 mm)
Circumference	80.36 in	83.76 in
Revolutions / mile	788	756

Other comparisons for these sizes

255/35R18 vs 275/35R18 255/40R18 vs 275/40R18 265/35R18 vs 275/35R18 275/35R18 vs 285/30R19 275/35R18 vs 275/30R19 275/40R18 vs 275/35R19 275/40R18 vs 295/30R20

FAQ

Frequently asked

What is the diameter difference between 275/35R18 and 275/40R18?: 275/35R18 has an overall diameter of 25.58 inches (650 mm). 275/40R18 is 26.66 inches (677 mm). That's a 1.08 inch (28 mm) difference — 275/40R18 is taller.
Will 275/40R18 cause speedometer error on a car fitted with 275/35R18?: Swapping 275/35R18 for 275/40R18 produces a speedometer error of about 4.2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.5 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 275/35R18 turns 788 revs per mile. 275/40R18 turns 756 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 275/35R18 has a sidewall of 96 mm (3.79 in). 275/40R18 sidewall is 110 mm (4.33 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/40R18 a safe plus-size upgrade from 275/35R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4.2%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.