TireSizesFinder

215/45R17 vs 205/40R18

205/40R18 is 0.16 in (4.1 mm) shorter than 215/45R17, a −0.66% diameter difference. With the speedometer set for 215/45R17, an indicated 60 mph is 59.6 mph on 205/40R18.

Is this a safe swap?

Diameter difference −0.66%: under 1%

By overall diameter alone, 205/40R18 is a near match for 215/45R17; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 215/45R17 (24.62 in) vs 205/40R18 (24.46 in)215/45R1724.62 in × 8.46 in205/40R1824.46 in × 8.07 in
Side profile and tread width, drawn to the same scale.
215/45R17 vs 205/40R18 differences
Measurement215/45R17205/40R18Difference% diff
Overall diameter24.62 in (625.3 mm)24.46 in (621.2 mm)−0.16 in (−4.1 mm)−0.66%
Sidewall height3.81 in (96.8 mm)3.23 in (82.0 mm)−0.58 in (−14.7 mm)−15.25%
Section width8.46 in (215.0 mm)8.07 in (205.0 mm)−0.39 in (−10.0 mm)−4.65%
Circumference77.34 in (1,964.4 mm)76.83 in (1,951.6 mm)−0.51 in (−12.9 mm)−0.66%
Revolutions per mile819825+5+0.66%
True speed with the speedometer calibrated for 215/45R17
IndicatedActual on 215/45R17Actual on 205/40R18Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.6 mph−0.4 mph
75 mph75.0 mph74.5 mph−0.5 mph

Ride-height change: 205/40R18 lowers the vehicle by 0.08 in (2.0 mm) at the axle (half the −4.1 mm diameter difference).

205/40R18 is 0.16 in shorter than 215/45R17 and 10 mm narrower, a 0.66% difference in overall diameter.

With the speedometer calibrated for 215/45R17, an indicated 60 mph is actually 59.6 mph on 205/40R18, and revolutions per mile change from 819 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/45R17.

Size pages

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Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.