See also: Prices and stock of individual 3½″ wheel castings.
I have two drawings for the tyre profile for 3½″ gauge. (See Form Tools). One is a reproduction of the BR profile at a scale of 1:16 (with a 1.8 mm deep flange), the other has a slightly deeper flange (at 2.4 mm) and is based on the dimensions on the I.B.L.S. web site for live steam models.
Tyre widths: Martin Evans suggests 9.5 mm which is almost exact scale width for a 6 inch wide tyre. IBLS suggest 10.32 mm (13/32″) minimum, presumably to allow for over-scale flange ways.
[See the suggested track and wheel standards for live steam models by the I.B.L.S (International Brotherhood of Live Steamers).]