275/40R17 vs 245/40R18

245/40R18 is 0.06 in (1.4 mm) taller than 275/40R17, a +0.21% diameter difference. With the speedometer set for 275/40R17, an indicated 60 mph is 60.1 mph on 245/40R18.

Is this a safe swap?

Diameter difference +0.21%: under 1%

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

275/40R17 vs 245/40R18 differences
Measurement	275/40R17	245/40R18	Difference	% diff
Overall diameter	25.66 in (651.8 mm)	25.72 in (653.2 mm)	+0.06 in (+1.4 mm)	+0.21%
Sidewall height	4.33 in (110.0 mm)	3.86 in (98.0 mm)	−0.47 in (−12.0 mm)	−10.91%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	80.62 in (2,047.7 mm)	80.79 in (2,052.1 mm)	+0.17 in (+4.4 mm)	+0.21%
Revolutions per mile	786	784	−2	−0.21%

True speed with the speedometer calibrated for 275/40R17
Indicated	Actual on 275/40R17	Actual on 245/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 245/40R18 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

245/40R18 is 0.06 in taller than 275/40R17 and 30 mm narrower, a 0.21% difference in overall diameter.

With the speedometer calibrated for 275/40R17, an indicated 60 mph is actually 60.1 mph on 245/40R18, and revolutions per mile change from 786 to 784.

245/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 275/40R17.

Size pages

Other comparisons with 275/40R17

Other comparisons with 245/40R18

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