265/50R18 vs 275/40R20

275/40R20 is 0.23 in (5.8 mm) taller than 265/50R18, a +0.80% diameter difference. With the speedometer set for 265/50R18, an indicated 60 mph is 60.5 mph on 275/40R20.

Is this a safe swap?

Diameter difference +0.80%: under 1%

By overall diameter alone, 275/40R20 is a near match for 265/50R18; 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.

265/50R18 vs 275/40R20 differences
Measurement	265/50R18	275/40R20	Difference	% diff
Overall diameter	28.43 in (722.2 mm)	28.66 in (728.0 mm)	+0.23 in (+5.8 mm)	+0.80%
Sidewall height	5.22 in (132.5 mm)	4.33 in (110.0 mm)	−0.89 in (−22.5 mm)	−16.98%
Section width	10.43 in (265.0 mm)	10.83 in (275.0 mm)	+0.39 in (+10.0 mm)	+3.77%
Circumference	89.33 in (2,268.9 mm)	90.04 in (2,287.1 mm)	+0.72 in (+18.2 mm)	+0.80%
Revolutions per mile	709	704	−6	−0.80%

True speed with the speedometer calibrated for 265/50R18
Indicated	Actual on 265/50R18	Actual on 275/40R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 275/40R20 raises the vehicle by 0.11 in (2.9 mm) at the axle (half the +5.8 mm diameter difference).

275/40R20 is 0.23 in taller than 265/50R18 and 10 mm wider, a 0.80% difference in overall diameter.

With the speedometer calibrated for 265/50R18, an indicated 60 mph is actually 60.5 mph on 275/40R20, and revolutions per mile change from 709 to 704.

275/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/50R18.

Size pages

Other comparisons with 265/50R18

Other comparisons with 275/40R20

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