235/40R18 vs 215/45R18

215/45R18 is 0.22 in (5.5 mm) taller than 235/40R18, a +0.85% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 60.5 mph on 215/45R18.

Is this a safe swap?

Diameter difference +0.85%: under 1%

By overall diameter alone, 215/45R18 is a near match for 235/40R18; 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.

235/40R18 vs 215/45R18 differences
Measurement	235/40R18	215/45R18	Difference	% diff
Overall diameter	25.40 in (645.2 mm)	25.62 in (650.7 mm)	+0.22 in (+5.5 mm)	+0.85%
Sidewall height	3.70 in (94.0 mm)	3.81 in (96.8 mm)	+0.11 in (+2.8 mm)	+2.93%
Section width	9.25 in (235.0 mm)	8.46 in (215.0 mm)	−0.79 in (−20.0 mm)	−8.51%
Circumference	79.80 in (2,027.0 mm)	80.48 in (2,044.2 mm)	+0.68 in (+17.3 mm)	+0.85%
Revolutions per mile	794	787	−7	−0.85%

True speed with the speedometer calibrated for 235/40R18
Indicated	Actual on 235/40R18	Actual on 215/45R18	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 215/45R18 raises the vehicle by 0.11 in (2.8 mm) at the axle (half the +5.5 mm diameter difference).

215/45R18 is 0.22 in taller than 235/40R18 and 20 mm narrower, a 0.85% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 60.5 mph on 215/45R18, and revolutions per mile change from 794 to 787.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/40R18

Other comparisons with 215/45R18

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