TireSizesFinder

205/55R17 vs 245/40R18

245/40R18 is 0.16 in (4.1 mm) shorter than 205/55R17, a −0.62% diameter difference. With the speedometer set for 205/55R17, an indicated 60 mph is 59.6 mph on 245/40R18.

Is this a safe swap?

Diameter difference −0.62%: under 1%

By overall diameter alone, 245/40R18 is a near match for 205/55R17; 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: 205/55R17 (25.88 in) vs 245/40R18 (25.72 in)205/55R1725.88 in × 8.07 in245/40R1825.72 in × 9.65 in
Side profile and tread width, drawn to the same scale.
205/55R17 vs 245/40R18 differences
Measurement205/55R17245/40R18Difference% diff
Overall diameter25.88 in (657.3 mm)25.72 in (653.2 mm)−0.16 in (−4.1 mm)−0.62%
Sidewall height4.44 in (112.8 mm)3.86 in (98.0 mm)−0.58 in (−14.7 mm)−13.08%
Section width8.07 in (205.0 mm)9.65 in (245.0 mm)+1.57 in (+40.0 mm)+19.51%
Circumference81.30 in (2,065.0 mm)80.79 in (2,052.1 mm)−0.51 in (−12.9 mm)−0.62%
Revolutions per mile779784+5+0.63%
True speed with the speedometer calibrated for 205/55R17
IndicatedActual on 205/55R17Actual on 245/40R18Difference
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: 245/40R18 lowers the vehicle by 0.08 in (2.0 mm) at the axle (half the −4.1 mm diameter difference).

245/40R18 is 0.16 in shorter than 205/55R17 and 40 mm wider, a 0.62% difference in overall diameter.

With the speedometer calibrated for 205/55R17, an indicated 60 mph is actually 59.6 mph on 245/40R18, and revolutions per mile change from 779 to 784.

245/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/55R17.

Size pages

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