255/35R21 vs 255/40R21

255/40R21 is 1.00 in (25.5 mm) taller than 255/35R21, a +3.58% diameter difference. With the speedometer set for 255/35R21, an indicated 60 mph is 62.1 mph on 255/40R21.

Is this a safe swap?

Diameter difference +3.58%: outside the ±3% guideline

By overall diameter alone, 255/40R21 falls outside the ±3% range commonly used as a guideline when replacing 255/35R21; speedometer and ride-height changes are larger. 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.

255/35R21 vs 255/40R21 differences
Measurement	255/35R21	255/40R21	Difference	% diff
Overall diameter	28.03 in (711.9 mm)	29.03 in (737.4 mm)	+1.00 in (+25.5 mm)	+3.58%
Sidewall height	3.51 in (89.3 mm)	4.02 in (102.0 mm)	+0.50 in (+12.8 mm)	+14.29%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	88.05 in (2,236.5 mm)	91.21 in (2,316.6 mm)	+3.15 in (+80.1 mm)	+3.58%
Revolutions per mile	720	695	−25	−3.46%

True speed with the speedometer calibrated for 255/35R21
Indicated	Actual on 255/35R21	Actual on 255/40R21	Difference
30 mph	30.0 mph	31.1 mph	+1.1 mph
60 mph	60.0 mph	62.1 mph	+2.1 mph
75 mph	75.0 mph	77.7 mph	+2.7 mph

Ride-height change: 255/40R21 raises the vehicle by 0.50 in (12.8 mm) at the axle (half the +25.5 mm diameter difference).

255/40R21 is 1.00 in taller than 255/35R21 and the same width, a 3.58% difference in overall diameter.

With the speedometer calibrated for 255/35R21, an indicated 60 mph is actually 62.1 mph on 255/40R21, and revolutions per mile change from 720 to 695.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 255/35R21

Other comparisons with 255/40R21

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