TireSizesFinder

225/50R16 vs 205/40R18

205/40R18 is 0.40 in (10.2 mm) shorter than 225/50R16, a −1.62% diameter difference. With the speedometer set for 225/50R16, an indicated 60 mph is 59.0 mph on 205/40R18.

Is this a safe swap?

Diameter difference −1.62%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 225/50R16. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 225/50R16 (24.86 in) vs 205/40R18 (24.46 in)225/50R1624.86 in × 8.86 in205/40R1824.46 in × 8.07 in
Side profile and tread width, drawn to the same scale.
225/50R16 vs 205/40R18 differences
Measurement225/50R16205/40R18Difference% diff
Overall diameter24.86 in (631.4 mm)24.46 in (621.2 mm)−0.40 in (−10.2 mm)−1.62%
Sidewall height4.43 in (112.5 mm)3.23 in (82.0 mm)−1.20 in (−30.5 mm)−27.11%
Section width8.86 in (225.0 mm)8.07 in (205.0 mm)−0.79 in (−20.0 mm)−8.89%
Circumference78.09 in (1,983.6 mm)76.83 in (1,951.6 mm)−1.26 in (−32.0 mm)−1.62%
Revolutions per mile811825+13+1.64%
True speed with the speedometer calibrated for 225/50R16
IndicatedActual on 225/50R16Actual on 205/40R18Difference
30 mph30.0 mph29.5 mph−0.5 mph
60 mph60.0 mph59.0 mph−1.0 mph
75 mph75.0 mph73.8 mph−1.2 mph

Ride-height change: 205/40R18 lowers the vehicle by 0.20 in (5.1 mm) at the axle (half the −10.2 mm diameter difference).

205/40R18 is 0.40 in shorter than 225/50R16 and 20 mm narrower, a 1.62% difference in overall diameter.

With the speedometer calibrated for 225/50R16, an indicated 60 mph is actually 59.0 mph on 205/40R18, and revolutions per mile change from 811 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 225/50R16.

Size pages

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