Tire Size Box

205/40R18 vs 205/50R18

205/50R18 compared with 205/40R18

+6.6% diameter, 0.0" width

205/50R18 is 26.07" tall vs 24.46". More than the 3% rule of thumb, so check carefully before switching.

24.5"18" rim3.2"8.1" wide
205/40R18 (solid) compared with 205/50R18 (dashed outline), drawn to scale

With 205/50R18 tires, when your speedometer says 60 mph you are really going

64.0 mph

205/50R18 is +1.61 inches (+6.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

205/40R18205/50R18Difference%
Overall diameter24.46"26.07"+1.61"+6.6%
Section width8.07"8.07"0.00"0.0%
Sidewall height3.23"4.04"+0.81"+25.0%
Circumference76.83"81.90"+5.07"+6.6%
Revolutions per mile824.6773.6−51.1−6.2%

Speedometer difference

If your vehicle came with 205/40R18 and you fit 205/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.6 mph+2.6 mph
50 mph53.3 mph+3.3 mph
60 mph64.0 mph+4.0 mph
65 mph69.3 mph+4.3 mph
70 mph74.6 mph+4.6 mph
75 mph80.0 mph+5.0 mph
80 mph85.3 mph+5.3 mph

Each size on its own

Questions

Which is bigger, 205/40R18 or 205/50R18?
205/50R18 is taller by 1.61 inches (26.07" vs 24.46"). 205/40R18 is the same width.
Can I replace 205/40R18 with 205/50R18?
It is not recommended without checking carefully. The diameter difference is +6.6%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 205/50R18 change my speedometer?
When your speedometer reads 60 mph with 205/50R18 fitted (calibrated for 205/40R18), your real speed is 64.0 mph. Your odometer will under-count miles by about 6.6%.
Will 205/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.81 inches of ground clearance at the axle.

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