Tire Size Box

315/40R18 vs 315/50R18

315/50R18 compared with 315/40R18

+8.9% diameter, 0.0" width

315/50R18 is 30.40" tall vs 27.92". More than the 3% rule of thumb, so check carefully before switching.

27.9"18" rim5.0"12.4" wide
315/40R18 (solid) compared with 315/50R18 (dashed outline), drawn to scale

With 315/50R18 tires, when your speedometer says 60 mph you are really going

65.3 mph

315/50R18 is +2.48 inches (+8.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R18315/50R18Difference%
Overall diameter27.92"30.40"+2.48"+8.9%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"6.20"+1.24"+25.0%
Circumference87.72"95.51"+7.79"+8.9%
Revolutions per mile722.3663.4−58.9−8.2%

Speedometer difference

If your vehicle came with 315/40R18 and you fit 315/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.4 mph+4.4 mph
60 mph65.3 mph+5.3 mph
65 mph70.8 mph+5.8 mph
70 mph76.2 mph+6.2 mph
75 mph81.7 mph+6.7 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 315/40R18 or 315/50R18?
315/50R18 is taller by 2.48 inches (30.40" vs 27.92"). 315/40R18 is the same width.
Can I replace 315/40R18 with 315/50R18?
It is not recommended without checking carefully. The diameter difference is +8.9%, 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 315/50R18 change my speedometer?
When your speedometer reads 60 mph with 315/50R18 fitted (calibrated for 315/40R18), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.9%.
Will 315/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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