Tire Size Box

315/40R17 vs 315/50R17

315/50R17 compared with 315/40R17

+9.2% diameter, 0.0" width

315/50R17 is 29.40" tall vs 26.92". More than the 3% rule of thumb, so check carefully before switching.

26.9"17" rim5.0"12.4" wide
315/40R17 (solid) compared with 315/50R17 (dashed outline), drawn to scale

With 315/50R17 tires, when your speedometer says 60 mph you are really going

65.5 mph

315/50R17 is +2.48 inches (+9.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R17315/50R17Difference%
Overall diameter26.92"29.40"+2.48"+9.2%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"6.20"+1.24"+25.0%
Circumference84.58"92.37"+7.79"+9.2%
Revolutions per mile749.2686.0−63.2−8.4%

Speedometer difference

If your vehicle came with 315/40R17 and you fit 315/50R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.8 mph+2.8 mph
40 mph43.7 mph+3.7 mph
50 mph54.6 mph+4.6 mph
60 mph65.5 mph+5.5 mph
65 mph71.0 mph+6.0 mph
70 mph76.4 mph+6.4 mph
75 mph81.9 mph+6.9 mph
80 mph87.4 mph+7.4 mph

Each size on its own

Questions

Which is bigger, 315/40R17 or 315/50R17?
315/50R17 is taller by 2.48 inches (29.40" vs 26.92"). 315/40R17 is the same width.
Can I replace 315/40R17 with 315/50R17?
It is not recommended without checking carefully. The diameter difference is +9.2%, 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 315/50R17 change my speedometer?
When your speedometer reads 60 mph with 315/50R17 fitted (calibrated for 315/40R17), your real speed is 65.5 mph. Your odometer will under-count miles by about 9.2%.
Will 315/50R17 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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