Tire Size Box

315/60R18 vs 33x12.50R18

33x12.50R18 compared with 315/60R18

+0.4% diameter, +0.1" width

33x12.50R18 is 33.00" tall vs 32.88". Within the 3% rule, so usually interchangeable.

32.9"18" rim7.4"12.4" wide
315/60R18 (solid) compared with 33x12.50R18 (dashed outline), drawn to scale

With 33x12.50R18 tires, when your speedometer says 60 mph you are really going

60.2 mph

33x12.50R18 is +0.12 inches (+0.4%) in overall diameter. That is within the 3% most shops and makers allow.

315/60R1833x12.50R18Difference%
Overall diameter32.88"33.00"+0.12"+0.4%
Section width12.40"12.50"+0.10"+0.8%
Sidewall height7.44"7.50"+0.06"+0.8%
Circumference103.30"103.67"+0.37"+0.4%
Revolutions per mile613.4611.2−2.2−0.4%

Speedometer difference

If your vehicle came with 315/60R18 and you fit 33x12.50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.1 mph+0.1 mph
30 mph30.1 mph+0.1 mph
40 mph40.1 mph+0.1 mph
50 mph50.2 mph+0.2 mph
60 mph60.2 mph+0.2 mph
65 mph65.2 mph+0.2 mph
70 mph70.3 mph+0.3 mph
75 mph75.3 mph+0.3 mph
80 mph80.3 mph+0.3 mph

Each size on its own

Questions

Which is bigger, 315/60R18 or 33x12.50R18?
33x12.50R18 is taller by 0.12 inches (33.00" vs 32.88"). 33x12.50R18 is wider by 0.10 inches.
Can I replace 315/60R18 with 33x12.50R18?
Usually, yes. The diameter difference is +0.4%, 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 33x12.50R18 change my speedometer?
When your speedometer reads 60 mph with 33x12.50R18 fitted (calibrated for 315/60R18), your real speed is 60.2 mph. Your odometer will under-count miles by about 0.4%.
Will 33x12.50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.06 inches of ground clearance at the axle.

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