Tire Size Box

285/40R18 vs 285/50R18

285/50R18 compared with 285/40R18

+8.3% diameter, 0.0" width

285/50R18 is 29.22" tall vs 26.98". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"11.2" wide
285/40R18 (solid) compared with 285/50R18 (dashed outline), drawn to scale

With 285/50R18 tires, when your speedometer says 60 mph you are really going

65.0 mph

285/50R18 is +2.24 inches (+8.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/40R18285/50R18Difference%
Overall diameter26.98"29.22"+2.24"+8.3%
Section width11.22"11.22"0.00"0.0%
Sidewall height4.49"5.61"+1.12"+25.0%
Circumference84.75"91.80"+7.05"+8.3%
Revolutions per mile747.6690.2−57.4−7.7%

Speedometer difference

If your vehicle came with 285/40R18 and you fit 285/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.2 mph+6.2 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 285/40R18 or 285/50R18?
285/50R18 is taller by 2.24 inches (29.22" vs 26.98"). 285/40R18 is the same width.
Can I replace 285/40R18 with 285/50R18?
It is not recommended without checking carefully. The diameter difference is +8.3%, 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 285/50R18 change my speedometer?
When your speedometer reads 60 mph with 285/50R18 fitted (calibrated for 285/40R18), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.3%.
Will 285/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.12 inches of ground clearance at the axle.

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