255/45R20 vs 295/35R21

295/35R21 is 0.09 in (2.4 mm) taller than 255/45R20, a +0.33% diameter difference. With the speedometer set for 255/45R20, an indicated 60 mph is 60.2 mph on 295/35R21.

Is this a safe swap?

Diameter difference +0.33%: under 1%

By overall diameter alone, 295/35R21 is a near match for 255/45R20; 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.

255/45R20 vs 295/35R21 differences
Measurement	255/45R20	295/35R21	Difference	% diff
Overall diameter	29.04 in (737.5 mm)	29.13 in (739.9 mm)	+0.09 in (+2.4 mm)	+0.33%
Sidewall height	4.52 in (114.8 mm)	4.06 in (103.3 mm)	−0.45 in (−11.5 mm)	−10.02%
Section width	10.04 in (255.0 mm)	11.61 in (295.0 mm)	+1.57 in (+40.0 mm)	+15.69%
Circumference	91.22 in (2,316.9 mm)	91.51 in (2,324.5 mm)	+0.30 in (+7.5 mm)	+0.33%
Revolutions per mile	695	692	−2	−0.32%

True speed with the speedometer calibrated for 255/45R20
Indicated	Actual on 255/45R20	Actual on 295/35R21	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	75.2 mph	+0.2 mph

Ride-height change: 295/35R21 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.4 mm diameter difference).

295/35R21 is 0.09 in taller than 255/45R20 and 40 mm wider, a 0.33% difference in overall diameter.

With the speedometer calibrated for 255/45R20, an indicated 60 mph is actually 60.2 mph on 295/35R21, and revolutions per mile change from 695 to 692.

295/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/45R20.

Size pages

Other comparisons with 255/45R20

Other comparisons with 295/35R21

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