49 C.F.R. § 230.112 – Wheels and tires.

(a) Mounting. Wheels shall be securely mounted on axles. Prick punching or shimming the wheel fit will not be permitted. The diameter of wheels on the same axle shall not vary more than 3/32 inch.
(b) Gage. Wheels used on standard gage track will be out of gage if the inside gage of flanges, measured on base line is less than 53 inches or more than 53 3/8 inches. Wheels used on less than standard gage track will be out of gage if the inside gage of flanges, measured on base line, is less than the relevant track gage less 3 1/2 inches or more than the relevant track gage less 3 1/8 inches.
(c) Flange distance variance. The distance back to back of flanges of wheels mounted on the same axle shall not vary more than 1/4 inch.

(d) Tire thickness. Wheels may not have tires with a minimum thickness less than that indicated in the table in this paragraph (d). When retaining rings are used, measurements of tires to be taken from the outside circumference of the ring, and the minimum thickness of tires may be as much below the limits specified earlier in this paragraph (d) as the tires extend between the retaining rings, provided it does not reduce the thickness of the tire to less than 1 1/8 inches from the throat of flange to the counterbore for the retaining rings. The required minimum thickness for tires, by wheel center diameter and weight per axle, is as follows:

Weight per axle (weight on drivers divided by number of pairs of driving wheels)	Diameter of wheel center (inches)	Minimum thickness (inches)
30,000 pounds and under	44 and under	11⁄4
	Over 44 to 50	15⁄16
	Over 50 to 56	13⁄8
	Over 56 to 62	17⁄16
	Over 62 to 68	11⁄2
	Over 68 to 74	19⁄16
	Over 74	15⁄8
Over 30,000 to 35,000 pounds	44 and under	15⁄16
	Over 44 to 50	13⁄8
	Over 50 to 56	17⁄16
	Over 56 to 62	11⁄2
	Over 62 to 68	19⁄16
	Over 68 to 74	15⁄8
	Over 74	111⁄16
Over 35,000 to 40,000 pounds	44 and under	13⁄8
	Over 44 to 50	17⁄16
	Over 50 to 56	11⁄2
	Over 56 to 62	19⁄16
	Over 62 to 68	15⁄8
	Over 68 to 74	111⁄16
	Over 74	13⁄4
Over 40,000 to 45,000 pounds	44 and under	17⁄16
	Over 44 to 50	11⁄2
	Over 50 to 56	19⁄16
	Over 56 to 62	15⁄8
	Over 62 to 68	111⁄16
	Over 68 to 74	13⁄4
	Over 74	113⁄16
Over 45,000 to 50,000 pounds	44 and under	11⁄2
	Over 44 to 50	19⁄16
	Over 50 to 56	15⁄8
	Over 56 to 62	111⁄16
	Over 62 to 68	13⁄4
	Over 68 to 74	113⁄16
	Over 74	17⁄8
Over 50,000 to 55,000 pounds	44 and under	19⁄16
	Over 44 to 50	15⁄8
	Over 50 to 56	111⁄16
	Over 56 to 62	13⁄4
	Over 62 to 68	113⁄16
	Over 68 to 74	17⁄8
	Over 74	115⁄16
Over 55,000 pounds	44 and under	15⁄8
	Over 44 to 50	111⁄16
	Over 50 to 56	13⁄4
	Over 56 to 62	113⁄16
	Over 62 to 68	17⁄8
	Over 68 to 74	115⁄16
	Over 74	2

(e) Tire width. Flanged tires shall be no less than 5 1/2 inches wide for standard gage and no less than 5 inches wide for narrow gage. Plain tires shall be no less than 6 inches wide for standard gage and no less than 5 1/2 inches wide for narrow gage.