245/35R21 vs 255/35R21

255/35R21 is 0.28 in (7.0 mm) taller than 245/35R21, a +0.99% diameter difference. With the speedometer set for 245/35R21, an indicated 60 mph is 60.6 mph on 255/35R21.

Is this a safe swap?

Diameter difference +0.99%: under 1%

By overall diameter alone, 255/35R21 is a near match for 245/35R21; 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.

245/35R21 vs 255/35R21 differences
Measurement	245/35R21	255/35R21	Difference	% diff
Overall diameter	27.75 in (704.9 mm)	28.03 in (711.9 mm)	+0.28 in (+7.0 mm)	+0.99%
Sidewall height	3.38 in (85.8 mm)	3.51 in (89.3 mm)	+0.14 in (+3.5 mm)	+4.08%
Section width	9.65 in (245.0 mm)	10.04 in (255.0 mm)	+0.39 in (+10.0 mm)	+4.08%
Circumference	87.19 in (2,214.5 mm)	88.05 in (2,236.5 mm)	+0.87 in (+22.0 mm)	+0.99%
Revolutions per mile	727	720	−7	−0.98%

True speed with the speedometer calibrated for 245/35R21
Indicated	Actual on 245/35R21	Actual on 255/35R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 255/35R21 raises the vehicle by 0.14 in (3.5 mm) at the axle (half the +7.0 mm diameter difference).

255/35R21 is 0.28 in taller than 245/35R21 and 10 mm wider, a 0.99% difference in overall diameter.

With the speedometer calibrated for 245/35R21, an indicated 60 mph is actually 60.6 mph on 255/35R21, and revolutions per mile change from 727 to 720.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 245/35R21

Other comparisons with 255/35R21

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