275/35R18 vs 245/40R18

245/40R18 is 0.14 in (3.5 mm) taller than 275/35R18, a +0.54% diameter difference. With the speedometer set for 275/35R18, an indicated 60 mph is 60.3 mph on 245/40R18.

Is this a safe swap?

Diameter difference +0.54%: under 1%

By overall diameter alone, 245/40R18 is a near match for 275/35R18; 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.

275/35R18 vs 245/40R18 differences
Measurement	275/35R18	245/40R18	Difference	% diff
Overall diameter	25.58 in (649.7 mm)	25.72 in (653.2 mm)	+0.14 in (+3.5 mm)	+0.54%
Sidewall height	3.79 in (96.3 mm)	3.86 in (98.0 mm)	+0.07 in (+1.8 mm)	+1.82%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	80.36 in (2,041.1 mm)	80.79 in (2,052.1 mm)	+0.43 in (+11.0 mm)	+0.54%
Revolutions per mile	788	784	−4	−0.54%

True speed with the speedometer calibrated for 275/35R18
Indicated	Actual on 275/35R18	Actual on 245/40R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 245/40R18 raises the vehicle by 0.07 in (1.8 mm) at the axle (half the +3.5 mm diameter difference).

245/40R18 is 0.14 in taller than 275/35R18 and 30 mm narrower, a 0.54% difference in overall diameter.

With the speedometer calibrated for 275/35R18, an indicated 60 mph is actually 60.3 mph on 245/40R18, and revolutions per mile change from 788 to 784.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 275/35R18

Other comparisons with 245/40R18

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