I'll 2nd the Imex foams for parking lot running. Damn things are a ton of fun. Not sure about for pure speed runs how they hold up. I have some that I've taken up to around 50mph, but I cant vouch for them not blowing apart at higher speeds than that. I have heard stories of the foam tearing away from the wheel at high speeds.
And BTW Mike... They start out at 5.25" tall. and wear donw to 3.2" when they are dead. Im sure your 4.25" was just a typo but just dont want anyone to get confused. I have a set of foams made for 1/8 bugies also. Those start out at 4.25" and are also great fun for on road parking lot racing. :027:
http://imexrc.com/detail.aspx?ID=688