245/40R17 vs 225/35R18

225/35R18 is 0.52 in (13.1 mm) shorter than 245/40R17, a −2.09% diameter difference. With the speedometer set for 245/40R17, an indicated 60 mph is 58.7 mph on 225/35R18.

Is this a safe swap?

Diameter difference −2.09%: within the ±3% guideline

By overall diameter alone, 225/35R18 falls inside the ±3% range commonly used as a guideline when replacing 245/40R17. 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.

245/40R17 vs 225/35R18 differences
Measurement	245/40R17	225/35R18	Difference	% diff
Overall diameter	24.72 in (627.8 mm)	24.20 in (614.7 mm)	−0.52 in (−13.1 mm)	−2.09%
Sidewall height	3.86 in (98.0 mm)	3.10 in (78.8 mm)	−0.76 in (−19.2 mm)	−19.64%
Section width	9.65 in (245.0 mm)	8.86 in (225.0 mm)	−0.79 in (−20.0 mm)	−8.16%
Circumference	77.65 in (1,972.3 mm)	76.03 in (1,931.1 mm)	−1.62 in (−41.2 mm)	−2.09%
Revolutions per mile	816	833	+17	+2.13%

True speed with the speedometer calibrated for 245/40R17
Indicated	Actual on 245/40R17	Actual on 225/35R18	Difference
30 mph	30.0 mph	29.4 mph	−0.6 mph
60 mph	60.0 mph	58.7 mph	−1.3 mph
75 mph	75.0 mph	73.4 mph	−1.6 mph

Ride-height change: 225/35R18 lowers the vehicle by 0.26 in (6.5 mm) at the axle (half the −13.1 mm diameter difference).

225/35R18 is 0.52 in shorter than 245/40R17 and 20 mm narrower, a 2.09% difference in overall diameter.

With the speedometer calibrated for 245/40R17, an indicated 60 mph is actually 58.7 mph on 225/35R18, and revolutions per mile change from 816 to 833.

225/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 245/40R17.

Size pages

Other comparisons with 245/40R17

Other comparisons with 225/35R18

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