I was thinking of putting a bare wheel on a tractor and locking the brake on the opposite wheel so only the bare wheel turns, and then marking the rotating wheel with a Sharpie, and then holding a hacksaw to the line as it spins. That's probably going to be pretty close to true. I haven't measured a 6-12 and a 7" rim yet, but I think there's enough there to make one 8.5" rim out of both of them. I happen to have a pair of each of those laying around...
