49 CFR §229.73

1. (a)The variation in the circumference of wheels on the same axle may not exceed 1/4 inch (two tape sizes) when applied or turned.
2. (b)The maximum variation in the diameter between any two wheel sets in a three-powered-axle truck may not exceed 3/4 inch, except that when shims are used at the journal box springs to compensate for wheel diameter variation, the maximum variation may not exceed 1 1/4 inch. The maximum variation in the diameter between any two wheel sets on different trucks on a locomotive that has three-powered-axle trucks may not exceed 1 1/4 inch. The diameter of a wheel set is the average diameter of the two wheels on an axle.
3. (c)On standard gauge locomotives, the distance between the inside gauge of the flanges on non-wide flange wheels may not be less than 53 inches or more than 53 1/2 inches. The distance between the inside gauge of the flanges on wide flange wheels may not be less than 53 inches or more than 53 1/4 inches.
4. (d)The distance back to back of flanges of wheels mounted on the same axle shall not vary more than 1/4 inch.