235/70R15 vs 255/70R15

255/70R15 is 1.10 in (28.0 mm) taller than 235/70R15, a +3.94% diameter difference. With the speedometer set for 235/70R15, an indicated 60 mph is 62.4 mph on 255/70R15.

Is this a safe swap?

Diameter difference +3.94%: outside the ±3% guideline

By overall diameter alone, 255/70R15 falls outside the ±3% range commonly used as a guideline when replacing 235/70R15; speedometer and ride-height changes are larger. 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.

235/70R15 vs 255/70R15 differences
Measurement	235/70R15	255/70R15	Difference	% diff
Overall diameter	27.95 in (710.0 mm)	29.06 in (738.0 mm)	+1.10 in (+28.0 mm)	+3.94%
Sidewall height	6.48 in (164.5 mm)	7.03 in (178.5 mm)	+0.55 in (+14.0 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	87.82 in (2,230.5 mm)	91.28 in (2,318.5 mm)	+3.46 in (+88.0 mm)	+3.94%
Revolutions per mile	722	694	−27	−3.79%

True speed with the speedometer calibrated for 235/70R15
Indicated	Actual on 235/70R15	Actual on 255/70R15	Difference
30 mph	30.0 mph	31.2 mph	+1.2 mph
60 mph	60.0 mph	62.4 mph	+2.4 mph
75 mph	75.0 mph	78.0 mph	+3.0 mph

Ride-height change: 255/70R15 raises the vehicle by 0.55 in (14.0 mm) at the axle (half the +28.0 mm diameter difference).

255/70R15 is 1.10 in taller than 235/70R15 and 20 mm wider, a 3.94% difference in overall diameter.

With the speedometer calibrated for 235/70R15, an indicated 60 mph is actually 62.4 mph on 255/70R15, and revolutions per mile change from 722 to 694.

Both sizes mount on 15-inch wheels.

Size pages

Other comparisons with 235/70R15

Other comparisons with 255/70R15

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