I've had fun (read, no alternative) playing with tire sizes (still has 4.1 gears) since I rely on used and hand me downs--whatever is available--195/50--205/50 and 225/50--Yokohamas and Hoosiers. I installed an M41 long ago - while still my daily driver-but have never used OD (successfully) on any of the tracks it's run--5800-6300rpm at the end of straights in 4th. Some of the ITB guys I've spoken to here run 4.80-5.12 gears and 8,000rpm. My goal is to dig up a later 140 rear with either 4.3 or 4.56 gears and I've put aside a limited slip from a 265. Bearings, seals and the retaining rings are too hard to find and expensive for the early 140's.