255/75R17 vs 255/70R18

255/70R18 is 0.00 in (0.1 mm) shorter than 255/75R17, a −0.01% diameter difference. With the speedometer set for 255/75R17, an indicated 60 mph is 60.0 mph on 255/70R18.

Is this a safe swap?

Diameter difference −0.01%: under 1%

By overall diameter alone, 255/70R18 is a near match for 255/75R17; 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.

255/75R17 vs 255/70R18 differences
Measurement	255/75R17	255/70R18	Difference	% diff
Overall diameter	32.06 in (814.3 mm)	32.06 in (814.2 mm)	0.00 in (−0.1 mm)	−0.01%
Sidewall height	7.53 in (191.3 mm)	7.03 in (178.5 mm)	−0.50 in (−12.7 mm)	−6.67%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	100.72 in (2,558.2 mm)	100.70 in (2,557.9 mm)	−0.01 in (−0.3 mm)	−0.01%
Revolutions per mile	629	629	0	+0.01%

True speed with the speedometer calibrated for 255/75R17
Indicated	Actual on 255/75R17	Actual on 255/70R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/70R18 does not change the vehicle at the axle (half the −0.1 mm diameter difference).

255/70R18 is the same height as 255/75R17 and the same width, a 0.01% difference in overall diameter.

With the speedometer calibrated for 255/75R17, an indicated 60 mph is actually 60.0 mph on 255/70R18, and revolutions per mile change from 629 to 629.

255/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/75R17.

Size pages

Other comparisons with 255/75R17

Other comparisons with 255/70R18

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