245/40R17 vs 205/40R18

205/40R18 is 0.26 in (6.6 mm) shorter than 245/40R17, a −1.05% diameter difference. With the speedometer set for 245/40R17, an indicated 60 mph is 59.4 mph on 205/40R18.

Is this a safe swap?

Diameter difference −1.05%: within the ±3% guideline

By overall diameter alone, 205/40R18 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 205/40R18 differences
Measurement	245/40R17	205/40R18	Difference	% diff
Overall diameter	24.72 in (627.8 mm)	24.46 in (621.2 mm)	−0.26 in (−6.6 mm)	−1.05%
Sidewall height	3.86 in (98.0 mm)	3.23 in (82.0 mm)	−0.63 in (−16.0 mm)	−16.33%
Section width	9.65 in (245.0 mm)	8.07 in (205.0 mm)	−1.57 in (−40.0 mm)	−16.33%
Circumference	77.65 in (1,972.3 mm)	76.83 in (1,951.6 mm)	−0.82 in (−20.7 mm)	−1.05%
Revolutions per mile	816	825	+9	+1.06%

True speed with the speedometer calibrated for 245/40R17
Indicated	Actual on 245/40R17	Actual on 205/40R18	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.4 mph	−0.6 mph
75 mph	75.0 mph	74.2 mph	−0.8 mph

Ride-height change: 205/40R18 lowers the vehicle by 0.13 in (3.3 mm) at the axle (half the −6.6 mm diameter difference).

205/40R18 is 0.26 in shorter than 245/40R17 and 40 mm narrower, a 1.05% difference in overall diameter.

With the speedometer calibrated for 245/40R17, an indicated 60 mph is actually 59.4 mph on 205/40R18, and revolutions per mile change from 816 to 825.

205/40R18 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 205/40R18

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