245/75R17 vs 255/65R18

255/65R18 is 0.42 in (10.6 mm) shorter than 245/75R17, a −1.33% diameter difference. With the speedometer set for 245/75R17, an indicated 60 mph is 59.2 mph on 255/65R18.

Is this a safe swap?

Diameter difference −1.33%: within the ±3% guideline

By overall diameter alone, 255/65R18 falls inside the ±3% range commonly used as a guideline when replacing 245/75R17. 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.

245/75R17 vs 255/65R18 differences
Measurement	245/75R17	255/65R18	Difference	% diff
Overall diameter	31.47 in (799.3 mm)	31.05 in (788.7 mm)	−0.42 in (−10.6 mm)	−1.33%
Sidewall height	7.23 in (183.8 mm)	6.53 in (165.8 mm)	−0.71 in (−18.0 mm)	−9.80%
Section width	9.65 in (245.0 mm)	10.04 in (255.0 mm)	+0.39 in (+10.0 mm)	+4.08%
Circumference	98.86 in (2,511.1 mm)	97.55 in (2,477.8 mm)	−1.31 in (−33.3 mm)	−1.33%
Revolutions per mile	641	650	+9	+1.34%

True speed with the speedometer calibrated for 245/75R17
Indicated	Actual on 245/75R17	Actual on 255/65R18	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.2 mph	−0.8 mph
75 mph	75.0 mph	74.0 mph	−1.0 mph

Ride-height change: 255/65R18 lowers the vehicle by 0.21 in (5.3 mm) at the axle (half the −10.6 mm diameter difference).

255/65R18 is 0.42 in shorter than 245/75R17 and 10 mm wider, a 1.33% difference in overall diameter.

With the speedometer calibrated for 245/75R17, an indicated 60 mph is actually 59.2 mph on 255/65R18, and revolutions per mile change from 641 to 650.

255/65R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 245/75R17.

Size pages

Other comparisons with 245/75R17

Other comparisons with 255/65R18

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