255/40R17 vs 245/35R18

245/35R18 is 0.28 in (7.1 mm) shorter than 255/40R17, a −1.12% diameter difference. With the speedometer set for 255/40R17, an indicated 60 mph is 59.3 mph on 245/35R18.

Is this a safe swap?

Diameter difference −1.12%: within the ±3% guideline

By overall diameter alone, 245/35R18 falls inside the ±3% range commonly used as a guideline when replacing 255/40R17. 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.

255/40R17 vs 245/35R18 differences
Measurement	255/40R17	245/35R18	Difference	% diff
Overall diameter	25.03 in (635.8 mm)	24.75 in (628.7 mm)	−0.28 in (−7.1 mm)	−1.12%
Sidewall height	4.02 in (102.0 mm)	3.38 in (85.8 mm)	−0.64 in (−16.2 mm)	−15.93%
Section width	10.04 in (255.0 mm)	9.65 in (245.0 mm)	−0.39 in (−10.0 mm)	−3.92%
Circumference	78.64 in (1,997.4 mm)	77.76 in (1,975.1 mm)	−0.88 in (−22.3 mm)	−1.12%
Revolutions per mile	806	815	+9	+1.13%

True speed with the speedometer calibrated for 255/40R17
Indicated	Actual on 255/40R17	Actual on 245/35R18	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.3 mph	−0.7 mph
75 mph	75.0 mph	74.2 mph	−0.8 mph

Ride-height change: 245/35R18 lowers the vehicle by 0.14 in (3.5 mm) at the axle (half the −7.1 mm diameter difference).

245/35R18 is 0.28 in shorter than 255/40R17 and 10 mm narrower, a 1.12% difference in overall diameter.

With the speedometer calibrated for 255/40R17, an indicated 60 mph is actually 59.3 mph on 245/35R18, and revolutions per mile change from 806 to 815.

245/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/40R17.

Size pages

Other comparisons with 255/40R17

Other comparisons with 245/35R18

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