According to my imperfect math, 3mm = 1'-0" scale (assuming I interpret this correctly)is 1:101.6, or just a bit smaller than the new scale I'm about to launch one of these days, D (for Dennis) scale, which would have a proportion of 1:100. :)

A scale width wheel at 1:101.6 would be .052 wide, so Code 72, 64, and 54 wheels would all be too wide. It appears that 3mm scale has specified overly coarse track standards, just like all the other "small" scales.

If you are committed to adapting HO scale wheels, it would appear that your choices these days would be Code 88 or Code 64 (P:87 wheels). The Code 64 wheels should work if you fill the flangeways to match the .014" flange depth. Of course, that's going to cause a problem with Code 72 wheels with their .020" deep flanges, but that's why railroad wheels are designed to NOT be flange bearing, which forces the wheel to be more than twice the width of the max. flangeway, plus the width of the frog point, which is exactly the same reason the NMRA won't adopt the Code 99 wheel as the standard for HO.

Dennis

It's an obsolete size. When the NMRA did the original work on the RP-25 recommended practice, they defined a series of proportional sized wheel contours, each intended to work with the common coarse standards of the different scales. These are defined here:

http://www.nmra.org/standards/sandrp/pdf/RP-25%202009.07.pdf

Code 72 was used for a while by people who anted a finer than common wheel in HO scale, although that use was not sanctioned by the NMRA because the wheel is too narrow to ride across the frog flangeways. More recently, work on the P:87 standard has encouraged some manufacturers to make exact scale width wheels, which are .064" wide, and therefore would be a "Code 64" wheel. Because of this, I suspect the Code 72 wheel is just going to fade away.

Dennis