215/45R18 vs 245/40R18

245/40R18 is 0.10 in (2.5 mm) taller than 215/45R18, a +0.38% diameter difference. With the speedometer set for 215/45R18, an indicated 60 mph is 60.2 mph on 245/40R18.

Is this a safe swap?

Diameter difference +0.38%: under 1%

By overall diameter alone, 245/40R18 is a near match for 215/45R18; 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.

215/45R18 vs 245/40R18 differences
Measurement	215/45R18	245/40R18	Difference	% diff
Overall diameter	25.62 in (650.7 mm)	25.72 in (653.2 mm)	+0.10 in (+2.5 mm)	+0.38%
Sidewall height	3.81 in (96.8 mm)	3.86 in (98.0 mm)	+0.05 in (+1.3 mm)	+1.29%
Section width	8.46 in (215.0 mm)	9.65 in (245.0 mm)	+1.18 in (+30.0 mm)	+13.95%
Circumference	80.48 in (2,044.2 mm)	80.79 in (2,052.1 mm)	+0.31 in (+7.9 mm)	+0.38%
Revolutions per mile	787	784	−3	−0.38%

True speed with the speedometer calibrated for 215/45R18
Indicated	Actual on 215/45R18	Actual on 245/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 245/40R18 raises the vehicle by 0.05 in (1.3 mm) at the axle (half the +2.5 mm diameter difference).

245/40R18 is 0.10 in taller than 215/45R18 and 30 mm wider, a 0.38% difference in overall diameter.

With the speedometer calibrated for 215/45R18, an indicated 60 mph is actually 60.2 mph on 245/40R18, and revolutions per mile change from 787 to 784.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 215/45R18

Other comparisons with 245/40R18

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