235/40R17 vs 215/35R18

215/35R18 is 0.48 in (12.1 mm) shorter than 235/40R17, a −1.95% diameter difference. With the speedometer set for 235/40R17, an indicated 60 mph is 58.8 mph on 215/35R18.

Is this a safe swap?

Diameter difference −1.95%: within the ±3% guideline

By overall diameter alone, 215/35R18 falls inside the ±3% range commonly used as a guideline when replacing 235/40R17. 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.

235/40R17 vs 215/35R18 differences
Measurement	235/40R17	215/35R18	Difference	% diff
Overall diameter	24.40 in (619.8 mm)	23.93 in (607.7 mm)	−0.48 in (−12.1 mm)	−1.95%
Sidewall height	3.70 in (94.0 mm)	2.96 in (75.3 mm)	−0.74 in (−18.7 mm)	−19.95%
Section width	9.25 in (235.0 mm)	8.46 in (215.0 mm)	−0.79 in (−20.0 mm)	−8.51%
Circumference	76.66 in (1,947.2 mm)	75.16 in (1,909.1 mm)	−1.50 in (−38.0 mm)	−1.95%
Revolutions per mile	827	843	+16	+1.99%

True speed with the speedometer calibrated for 235/40R17
Indicated	Actual on 235/40R17	Actual on 215/35R18	Difference
30 mph	30.0 mph	29.4 mph	−0.6 mph
60 mph	60.0 mph	58.8 mph	−1.2 mph
75 mph	75.0 mph	73.5 mph	−1.5 mph

Ride-height change: 215/35R18 lowers the vehicle by 0.24 in (6.0 mm) at the axle (half the −12.1 mm diameter difference).

215/35R18 is 0.48 in shorter than 235/40R17 and 20 mm narrower, a 1.95% difference in overall diameter.

With the speedometer calibrated for 235/40R17, an indicated 60 mph is actually 58.8 mph on 215/35R18, and revolutions per mile change from 827 to 843.

215/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/40R17.

Size pages

Other comparisons with 235/40R17

Other comparisons with 215/35R18

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