245/40R21 vs 285/35R21

285/35R21 is 0.14 in (3.5 mm) taller than 245/40R21, a +0.48% diameter difference. With the speedometer set for 245/40R21, an indicated 60 mph is 60.3 mph on 285/35R21.

Is this a safe swap?

Diameter difference +0.48%: under 1%

By overall diameter alone, 285/35R21 is a near match for 245/40R21; 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.

245/40R21 vs 285/35R21 differences
Measurement	245/40R21	285/35R21	Difference	% diff
Overall diameter	28.72 in (729.4 mm)	28.85 in (732.9 mm)	+0.14 in (+3.5 mm)	+0.48%
Sidewall height	3.86 in (98.0 mm)	3.93 in (99.8 mm)	+0.07 in (+1.8 mm)	+1.79%
Section width	9.65 in (245.0 mm)	11.22 in (285.0 mm)	+1.57 in (+40.0 mm)	+16.33%
Circumference	90.22 in (2,291.5 mm)	90.65 in (2,302.5 mm)	+0.43 in (+11.0 mm)	+0.48%
Revolutions per mile	702	699	−3	−0.48%

True speed with the speedometer calibrated for 245/40R21
Indicated	Actual on 245/40R21	Actual on 285/35R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 285/35R21 raises the vehicle by 0.07 in (1.8 mm) at the axle (half the +3.5 mm diameter difference).

285/35R21 is 0.14 in taller than 245/40R21 and 40 mm wider, a 0.48% difference in overall diameter.

With the speedometer calibrated for 245/40R21, an indicated 60 mph is actually 60.3 mph on 285/35R21, and revolutions per mile change from 702 to 699.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 245/40R21

Other comparisons with 285/35R21

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