255/40R20 vs 265/35R21

265/35R21 is 0.27 in (6.9 mm) taller than 255/40R20, a +0.97% diameter difference. With the speedometer set for 255/40R20, an indicated 60 mph is 60.6 mph on 265/35R21.

Is this a safe swap?

Diameter difference +0.97%: under 1%

By overall diameter alone, 265/35R21 is a near match for 255/40R20; 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.

255/40R20 vs 265/35R21 differences
Measurement	255/40R20	265/35R21	Difference	% diff
Overall diameter	28.03 in (712.0 mm)	28.30 in (718.9 mm)	+0.27 in (+6.9 mm)	+0.97%
Sidewall height	4.02 in (102.0 mm)	3.65 in (92.8 mm)	−0.36 in (−9.2 mm)	−9.07%
Section width	10.04 in (255.0 mm)	10.43 in (265.0 mm)	+0.39 in (+10.0 mm)	+3.92%
Circumference	88.06 in (2,236.8 mm)	88.92 in (2,258.5 mm)	+0.85 in (+21.7 mm)	+0.97%
Revolutions per mile	719	713	−7	−0.96%

True speed with the speedometer calibrated for 255/40R20
Indicated	Actual on 255/40R20	Actual on 265/35R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 265/35R21 raises the vehicle by 0.14 in (3.4 mm) at the axle (half the +6.9 mm diameter difference).

265/35R21 is 0.27 in taller than 255/40R20 and 10 mm wider, a 0.97% difference in overall diameter.

With the speedometer calibrated for 255/40R20, an indicated 60 mph is actually 60.6 mph on 265/35R21, and revolutions per mile change from 719 to 713.

265/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/40R20.

Size pages

Other comparisons with 255/40R20

Other comparisons with 265/35R21

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