245/50R17 vs 265/40R18

265/40R18 is 0.30 in (7.6 mm) shorter than 245/50R17, a −1.12% diameter difference. With the speedometer set for 245/50R17, an indicated 60 mph is 59.3 mph on 265/40R18.

Is this a safe swap?

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

By overall diameter alone, 265/40R18 falls inside the ±3% range commonly used as a guideline when replacing 245/50R17. 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.

245/50R17 vs 265/40R18 differences
Measurement	245/50R17	265/40R18	Difference	% diff
Overall diameter	26.65 in (676.8 mm)	26.35 in (669.2 mm)	−0.30 in (−7.6 mm)	−1.12%
Sidewall height	4.82 in (122.5 mm)	4.17 in (106.0 mm)	−0.65 in (−16.5 mm)	−13.47%
Section width	9.65 in (245.0 mm)	10.43 in (265.0 mm)	+0.79 in (+20.0 mm)	+8.16%
Circumference	83.71 in (2,126.2 mm)	82.77 in (2,102.4 mm)	−0.94 in (−23.9 mm)	−1.12%
Revolutions per mile	757	765	+9	+1.14%

True speed with the speedometer calibrated for 245/50R17
Indicated	Actual on 245/50R17	Actual on 265/40R18	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: 265/40R18 lowers the vehicle by 0.15 in (3.8 mm) at the axle (half the −7.6 mm diameter difference).

265/40R18 is 0.30 in shorter than 245/50R17 and 20 mm wider, a 1.12% difference in overall diameter.

With the speedometer calibrated for 245/50R17, an indicated 60 mph is actually 59.3 mph on 265/40R18, and revolutions per mile change from 757 to 765.

265/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 245/50R17.

Size pages

Other comparisons with 245/50R17

Other comparisons with 265/40R18

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