RE: Chainsaw Powered Car
About your tire quandary, I guess I would ask if you're thinking of adding suspension as well, as I don't think you mentioned it. From the looks of it I guess you're going for a solid rear end? I went a fairly simply route and used some T-Maxx parts that you can see in one thread on my car [link=http://www.rcuniverse.com/forum/m_8272667/anchors_8272667/mpage_1/key_/anchor/tm.htm#8272667]here[/link]. If you machine your own parts, you can obviously make them sturdier, but this was fairly straightforward to mount up.
I believe your 4:1 gear reduction should still be relatively fast as I had way more reduction in an old car I had and it was decent. The car in the link I gave is about 7.5:1 reduction...