Tire Size Box

335/45R18 vs 315/50R18

315/50R18 compared with 335/45R18

+1.8% diameter, −0.8" width

315/50R18 is 30.40" tall vs 29.87". Within the 3% rule, so usually interchangeable.

29.9"18" rim5.9"13.2" wide
335/45R18 (solid) compared with 315/50R18 (dashed outline), drawn to scale

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

61.1 mph

315/50R18 is +0.53 inches (+1.8%) in overall diameter. That is within the 3% most shops and makers allow.

335/45R18315/50R18Difference%
Overall diameter29.87"30.40"+0.53"+1.8%
Section width13.19"12.40"−0.79"−6.0%
Sidewall height5.94"6.20"+0.27"+4.5%
Circumference93.84"95.51"+1.67"+1.8%
Revolutions per mile675.2663.4−11.8−1.7%

Speedometer difference

If your vehicle came with 335/45R18 and you fit 315/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.4 mph+0.4 mph
30 mph30.5 mph+0.5 mph
40 mph40.7 mph+0.7 mph
50 mph50.9 mph+0.9 mph
60 mph61.1 mph+1.1 mph
65 mph66.2 mph+1.2 mph
70 mph71.2 mph+1.2 mph
75 mph76.3 mph+1.3 mph
80 mph81.4 mph+1.4 mph

Each size on its own

Questions

Which is bigger, 335/45R18 or 315/50R18?
315/50R18 is taller by 0.53 inches (30.40" vs 29.87"). 335/45R18 is wider by 0.79 inches.
Can I replace 335/45R18 with 315/50R18?
Usually, yes. The diameter difference is +1.8%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 315/50R18 change my speedometer?
When your speedometer reads 60 mph with 315/50R18 fitted (calibrated for 335/45R18), your real speed is 61.1 mph. Your odometer will under-count miles by about 1.8%.
Will 315/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.27 inches of ground clearance at the axle.

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