245/40R21 vs 255/40R21

255/40R21 is 0.31 in (8.0 mm) taller than 245/40R21, a +1.10% diameter difference. With the speedometer set for 245/40R21, an indicated 60 mph is 60.7 mph on 255/40R21.

Is this a safe swap?

Diameter difference +1.10%: within the ±3% guideline

By overall diameter alone, 255/40R21 falls inside the ±3% range commonly used as a guideline when replacing 245/40R21. 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 255/40R21 differences
Measurement	245/40R21	255/40R21	Difference	% diff
Overall diameter	28.72 in (729.4 mm)	29.03 in (737.4 mm)	+0.31 in (+8.0 mm)	+1.10%
Sidewall height	3.86 in (98.0 mm)	4.02 in (102.0 mm)	+0.16 in (+4.0 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	90.22 in (2,291.5 mm)	91.21 in (2,316.6 mm)	+0.99 in (+25.1 mm)	+1.10%
Revolutions per mile	702	695	−8	−1.08%

True speed with the speedometer calibrated for 245/40R21
Indicated	Actual on 245/40R21	Actual on 255/40R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.8 mph	+0.8 mph

Ride-height change: 255/40R21 raises the vehicle by 0.16 in (4.0 mm) at the axle (half the +8.0 mm diameter difference).

255/40R21 is 0.31 in taller than 245/40R21 and 10 mm wider, a 1.10% difference in overall diameter.

With the speedometer calibrated for 245/40R21, an indicated 60 mph is actually 60.7 mph on 255/40R21, and revolutions per mile change from 702 to 695.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 245/40R21

Other comparisons with 255/40R21

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