235/80R17 vs 255/70R18

255/70R18 is 0.25 in (6.4 mm) taller than 235/80R17, a +0.79% diameter difference. With the speedometer set for 235/80R17, an indicated 60 mph is 60.5 mph on 255/70R18.

Is this a safe swap?

Diameter difference +0.79%: under 1%

By overall diameter alone, 255/70R18 is a near match for 235/80R17; 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.

235/80R17 vs 255/70R18 differences
Measurement	235/80R17	255/70R18	Difference	% diff
Overall diameter	31.80 in (807.8 mm)	32.06 in (814.2 mm)	+0.25 in (+6.4 mm)	+0.79%
Sidewall height	7.40 in (188.0 mm)	7.03 in (178.5 mm)	−0.37 in (−9.5 mm)	−5.05%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	99.91 in (2,537.8 mm)	100.70 in (2,557.9 mm)	+0.79 in (+20.1 mm)	+0.79%
Revolutions per mile	634	629	−5	−0.79%

True speed with the speedometer calibrated for 235/80R17
Indicated	Actual on 235/80R17	Actual on 255/70R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 255/70R18 raises the vehicle by 0.13 in (3.2 mm) at the axle (half the +6.4 mm diameter difference).

255/70R18 is 0.25 in taller than 235/80R17 and 20 mm wider, a 0.79% difference in overall diameter.

With the speedometer calibrated for 235/80R17, an indicated 60 mph is actually 60.5 mph on 255/70R18, and revolutions per mile change from 634 to 629.

255/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/80R17.

Size pages

Other comparisons with 235/80R17

Other comparisons with 255/70R18

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