255/50R20 vs 295/40R21

295/40R21 is 0.25 in (6.4 mm) taller than 255/50R20, a +0.84% diameter difference. With the speedometer set for 255/50R20, an indicated 60 mph is 60.5 mph on 295/40R21.

Is this a safe swap?

Diameter difference +0.84%: under 1%

By overall diameter alone, 295/40R21 is a near match for 255/50R20; 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.

255/50R20 vs 295/40R21 differences
Measurement	255/50R20	295/40R21	Difference	% diff
Overall diameter	30.04 in (763.0 mm)	30.29 in (769.4 mm)	+0.25 in (+6.4 mm)	+0.84%
Sidewall height	5.02 in (127.5 mm)	4.65 in (118.0 mm)	−0.37 in (−9.5 mm)	−7.45%
Section width	10.04 in (255.0 mm)	11.61 in (295.0 mm)	+1.57 in (+40.0 mm)	+15.69%
Circumference	94.37 in (2,397.0 mm)	95.16 in (2,417.1 mm)	+0.79 in (+20.1 mm)	+0.84%
Revolutions per mile	671	666	−6	−0.83%

True speed with the speedometer calibrated for 255/50R20
Indicated	Actual on 255/50R20	Actual on 295/40R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 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: 295/40R21 raises the vehicle by 0.13 in (3.2 mm) at the axle (half the +6.4 mm diameter difference).

295/40R21 is 0.25 in taller than 255/50R20 and 40 mm wider, a 0.84% difference in overall diameter.

With the speedometer calibrated for 255/50R20, an indicated 60 mph is actually 60.5 mph on 295/40R21, and revolutions per mile change from 671 to 666.

295/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/50R20.

Size pages

Other comparisons with 255/50R20

Other comparisons with 295/40R21

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