255/35R21 vs 275/35R21

275/35R21 is 0.55 in (14.0 mm) taller than 255/35R21, a +1.97% diameter difference. With the speedometer set for 255/35R21, an indicated 60 mph is 61.2 mph on 275/35R21.

Is this a safe swap?

Diameter difference +1.97%: within the ±3% guideline

By overall diameter alone, 275/35R21 falls inside the ±3% range commonly used as a guideline when replacing 255/35R21. 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 275/35R21 differences
Measurement	255/35R21	275/35R21	Difference	% diff
Overall diameter	28.03 in (711.9 mm)	28.58 in (725.9 mm)	+0.55 in (+14.0 mm)	+1.97%
Sidewall height	3.51 in (89.3 mm)	3.79 in (96.3 mm)	+0.28 in (+7.0 mm)	+7.84%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	88.05 in (2,236.5 mm)	89.78 in (2,280.5 mm)	+1.73 in (+44.0 mm)	+1.97%
Revolutions per mile	720	706	−14	−1.93%

True speed with the speedometer calibrated for 255/35R21
Indicated	Actual on 255/35R21	Actual on 275/35R21	Difference
30 mph	30.0 mph	30.6 mph	+0.6 mph
60 mph	60.0 mph	61.2 mph	+1.2 mph
75 mph	75.0 mph	76.5 mph	+1.5 mph

Ride-height change: 275/35R21 raises the vehicle by 0.28 in (7.0 mm) at the axle (half the +14.0 mm diameter difference).

275/35R21 is 0.55 in taller than 255/35R21 and 20 mm wider, a 1.97% difference in overall diameter.

With the speedometer calibrated for 255/35R21, an indicated 60 mph is actually 61.2 mph on 275/35R21, and revolutions per mile change from 720 to 706.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 255/35R21

Other comparisons with 275/35R21

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