245/50R17 vs 275/40R18

275/40R18 is 0.02 in (0.4 mm) taller than 245/50R17, a +0.06% diameter difference. With the speedometer set for 245/50R17, an indicated 60 mph is 60.0 mph on 275/40R18.

Is this a safe swap?

Diameter difference +0.06%: under 1%

By overall diameter alone, 275/40R18 is a near match for 245/50R17; 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.

245/50R17 vs 275/40R18 differences
Measurement	245/50R17	275/40R18	Difference	% diff
Overall diameter	26.65 in (676.8 mm)	26.66 in (677.2 mm)	+0.02 in (+0.4 mm)	+0.06%
Sidewall height	4.82 in (122.5 mm)	4.33 in (110.0 mm)	−0.49 in (−12.5 mm)	−10.20%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	83.71 in (2,126.2 mm)	83.76 in (2,127.5 mm)	+0.05 in (+1.3 mm)	+0.06%
Revolutions per mile	757	756	0	−0.06%

True speed with the speedometer calibrated for 245/50R17
Indicated	Actual on 245/50R17	Actual on 275/40R18
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: 275/40R18 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

275/40R18 is 0.02 in taller than 245/50R17 and 30 mm wider, a 0.06% difference in overall diameter.

With the speedometer calibrated for 245/50R17, an indicated 60 mph is actually 60.0 mph on 275/40R18, and revolutions per mile change from 757 to 756.

275/40R18 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 275/40R18

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