275/40R18 vs 245/35R20

245/35R20 is 0.09 in (2.3 mm) taller than 275/40R18, a +0.34% diameter difference. With the speedometer set for 275/40R18, an indicated 60 mph is 60.2 mph on 245/35R20.

Is this a safe swap?

Diameter difference +0.34%: under 1%

By overall diameter alone, 245/35R20 is a near match for 275/40R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

275/40R18 vs 245/35R20 differences
Measurement	275/40R18	245/35R20	Difference	% diff
Overall diameter	26.66 in (677.2 mm)	26.75 in (679.5 mm)	+0.09 in (+2.3 mm)	+0.34%
Sidewall height	4.33 in (110.0 mm)	3.38 in (85.8 mm)	−0.95 in (−24.2 mm)	−22.05%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	83.76 in (2,127.5 mm)	84.04 in (2,134.7 mm)	+0.28 in (+7.2 mm)	+0.34%
Revolutions per mile	756	754	−3	−0.34%

True speed with the speedometer calibrated for 275/40R18
Indicated	Actual on 275/40R18	Actual on 245/35R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 245/35R20 raises the vehicle by 0.05 in (1.1 mm) at the axle (half the +2.3 mm diameter difference).

245/35R20 is 0.09 in taller than 275/40R18 and 30 mm narrower, a 0.34% difference in overall diameter.

With the speedometer calibrated for 275/40R18, an indicated 60 mph is actually 60.2 mph on 245/35R20, and revolutions per mile change from 756 to 754.

245/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/40R18.

Size pages

Other comparisons with 275/40R18

Other comparisons with 245/35R20

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.