Tire Size Box

37x12.50R20 vs 37x13.50R20

37x13.50R20 compared with 37x12.50R20

0.0% diameter, +1.0" width

37x13.50R20 is 37.00" tall vs 37.00". Within the 3% rule, so usually interchangeable.

37.0"20" rim8.5"12.5" wide
37x12.50R20 (solid) compared with 37x13.50R20 (dashed outline), drawn to scale

With 37x13.50R20 tires, when your speedometer says 60 mph you are really going

60.0 mph

37x13.50R20 is 0.00 inches (0.0%) in overall diameter. That is within the 3% most shops and makers allow.

37x12.50R2037x13.50R20Difference%
Overall diameter37.00"37.00"0.00"0.0%
Section width12.50"13.50"+1.00"+8.0%
Sidewall height8.50"8.50"0.00"0.0%
Circumference116.24"116.24"0.00"0.0%
Revolutions per mile545.1545.10.00.0%

Speedometer difference

If your vehicle came with 37x12.50R20 and you fit 37x13.50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.0 mph0.0 mph
40 mph40.0 mph0.0 mph
50 mph50.0 mph0.0 mph
60 mph60.0 mph0.0 mph
65 mph65.0 mph0.0 mph
70 mph70.0 mph0.0 mph
75 mph75.0 mph0.0 mph
80 mph80.0 mph0.0 mph

Each size on its own

Questions

Which is bigger, 37x12.50R20 or 37x13.50R20?
They are the same overall diameter, 37.00 inches.
Can I replace 37x12.50R20 with 37x13.50R20?
Usually, yes. The diameter difference is 0.0%, 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 37x13.50R20 change my speedometer?
When your speedometer reads 60 mph with 37x13.50R20 fitted (calibrated for 37x12.50R20), your real speed is 60.0 mph. Your odometer will under-count miles by about 0.0%.
Will 37x13.50R20 lift my vehicle?
Ride height changes by half the diameter difference: 0.00 inches of ground clearance at the axle.

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