295/30R21 vs 255/35R21

255/35R21 is 0.06 in (1.5 mm) taller than 295/30R21, a +0.21% diameter difference. With the speedometer set for 295/30R21, an indicated 60 mph is 60.1 mph on 255/35R21.

Is this a safe swap?

Diameter difference +0.21%: under 1%

By overall diameter alone, 255/35R21 is a near match for 295/30R21; 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.

295/30R21 vs 255/35R21 differences
Measurement	295/30R21	255/35R21	Difference	% diff
Overall diameter	27.97 in (710.4 mm)	28.03 in (711.9 mm)	+0.06 in (+1.5 mm)	+0.21%
Sidewall height	3.48 in (88.5 mm)	3.51 in (89.3 mm)	+0.03 in (+0.8 mm)	+0.85%
Section width	11.61 in (295.0 mm)	10.04 in (255.0 mm)	−1.57 in (−40.0 mm)	−13.56%
Circumference	87.87 in (2,231.8 mm)	88.05 in (2,236.5 mm)	+0.19 in (+4.7 mm)	+0.21%
Revolutions per mile	721	720	−2	−0.21%

True speed with the speedometer calibrated for 295/30R21
Indicated	Actual on 295/30R21	Actual on 255/35R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 255/35R21 raises the vehicle by 0.03 in (0.8 mm) at the axle (half the +1.5 mm diameter difference).

255/35R21 is 0.06 in taller than 295/30R21 and 40 mm narrower, a 0.21% difference in overall diameter.

With the speedometer calibrated for 295/30R21, an indicated 60 mph is actually 60.1 mph on 255/35R21, and revolutions per mile change from 721 to 720.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 295/30R21

Other comparisons with 255/35R21

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