TireSizesFinder

205/50R17 vs 215/40R18

215/40R18 is 0.30 in (7.6 mm) shorter than 205/50R17, a −1.19% diameter difference. With the speedometer set for 205/50R17, an indicated 60 mph is 59.3 mph on 215/40R18.

Is this a safe swap?

Diameter difference −1.19%: within the ±3% guideline

By overall diameter alone, 215/40R18 falls inside the ±3% range commonly used as a guideline when replacing 205/50R17. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 205/50R17 (25.07 in) vs 215/40R18 (24.77 in)205/50R1725.07 in × 8.07 in215/40R1824.77 in × 8.46 in
Side profile and tread width, drawn to the same scale.
205/50R17 vs 215/40R18 differences
Measurement205/50R17215/40R18Difference% diff
Overall diameter25.07 in (636.8 mm)24.77 in (629.2 mm)−0.30 in (−7.6 mm)−1.19%
Sidewall height4.04 in (102.5 mm)3.39 in (86.0 mm)−0.65 in (−16.5 mm)−16.10%
Section width8.07 in (205.0 mm)8.46 in (215.0 mm)+0.39 in (+10.0 mm)+4.88%
Circumference78.76 in (2,000.6 mm)77.82 in (1,976.7 mm)−0.94 in (−23.9 mm)−1.19%
Revolutions per mile804814+10+1.21%
True speed with the speedometer calibrated for 205/50R17
IndicatedActual on 205/50R17Actual on 215/40R18Difference
30 mph30.0 mph29.6 mph−0.4 mph
60 mph60.0 mph59.3 mph−0.7 mph
75 mph75.0 mph74.1 mph−0.9 mph

Ride-height change: 215/40R18 lowers the vehicle by 0.15 in (3.8 mm) at the axle (half the −7.6 mm diameter difference).

215/40R18 is 0.30 in shorter than 205/50R17 and 10 mm wider, a 1.19% difference in overall diameter.

With the speedometer calibrated for 205/50R17, an indicated 60 mph is actually 59.3 mph on 215/40R18, and revolutions per mile change from 804 to 814.

215/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/50R17.

Size pages

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