Instead of swapping to a 6 speed MT, why not just change the gears in the rear differential? This would lower the RPM and save on gas. Is this even possible?? What are the advantages/disadvantages of doing this?
Most people change to the 6 speed because it is much stronger and can hold a lot more power. If you do put in lets say the 3.2 final drive from the supra in to the IS300 and keep it N/A, it would take 40 years for it to get to 60, top speed (if it can do it) would be like 180 or something , but you would never make it up there. It's just not practicle to do it. Now a 6 speed with the 3.9 final drive, would be nice, but it costs too much.