235/45R21 vs 255/45R21

255/45R21 is 0.71 in (18.0 mm) taller than 235/45R21, a +2.42% diameter difference. With the speedometer set for 235/45R21, an indicated 60 mph is 61.4 mph on 255/45R21.

Is this a safe swap?

Diameter difference +2.42%: within the ±3% guideline

By overall diameter alone, 255/45R21 falls inside the ±3% range commonly used as a guideline when replacing 235/45R21. 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.

235/45R21 vs 255/45R21 differences
Measurement	235/45R21	255/45R21	Difference	% diff
Overall diameter	29.33 in (744.9 mm)	30.04 in (762.9 mm)	+0.71 in (+18.0 mm)	+2.42%
Sidewall height	4.16 in (105.8 mm)	4.52 in (114.8 mm)	+0.35 in (+9.0 mm)	+8.51%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	92.13 in (2,340.2 mm)	94.36 in (2,396.7 mm)	+2.23 in (+56.5 mm)	+2.42%
Revolutions per mile	688	671	−16	−2.36%

True speed with the speedometer calibrated for 235/45R21
Indicated	Actual on 235/45R21	Actual on 255/45R21	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.4 mph	+1.4 mph
75 mph	75.0 mph	76.8 mph	+1.8 mph

Ride-height change: 255/45R21 raises the vehicle by 0.35 in (9.0 mm) at the axle (half the +18.0 mm diameter difference).

255/45R21 is 0.71 in taller than 235/45R21 and 20 mm wider, a 2.42% difference in overall diameter.

With the speedometer calibrated for 235/45R21, an indicated 60 mph is actually 61.4 mph on 255/45R21, and revolutions per mile change from 688 to 671.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 235/45R21

Other comparisons with 255/45R21

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