df-03 Wheel options

[gracoat]

Hey everyone... I'm more of a back yard/kids school's gravel field racer than a stadium racer. The tires that are on my df-03 would do well on an on road car after a few months of bombing around. There's practically no tread left on them at all.

Two questions:
1: It seems that every tire on the market is a slightly different size and therefore I'm not sure that if I buy a set, they'll fit on my rims. I know as a 4 wheel drive car they gotta be the same diameter, but what would happen if I bought a set of wheels from like a bigger truggy style vehicle with rims? Gunna fit?

and 2: The rims that are on my car have this hex thingy the rim meets up with that's on the axle. Is this hex thingy standard equipment on rims or is it a Tamiya proprietary idea?