Tire Size Box

315/45R18 vs 315/50R18

315/50R18 compared with 315/45R18

+4.3% diameter, 0.0" width

315/50R18 is 30.40" tall vs 29.16". More than the 3% rule of thumb, so check carefully before switching.

29.2"18" rim5.6"12.4" wide
315/45R18 (solid) compared with 315/50R18 (dashed outline), drawn to scale

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

62.6 mph

315/50R18 is +1.24 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/45R18315/50R18Difference%
Overall diameter29.16"30.40"+1.24"+4.3%
Section width12.40"12.40"0.00"0.0%
Sidewall height5.58"6.20"+0.62"+11.1%
Circumference91.61"95.51"+3.90"+4.3%
Revolutions per mile691.6663.4−28.2−4.1%

Speedometer difference

If your vehicle came with 315/45R18 and you fit 315/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 315/45R18 or 315/50R18?
315/50R18 is taller by 1.24 inches (30.40" vs 29.16"). 315/45R18 is the same width.
Can I replace 315/45R18 with 315/50R18?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 315/50R18 change my speedometer?
When your speedometer reads 60 mph with 315/50R18 fitted (calibrated for 315/45R18), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 315/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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