Mark, thanks for the input. The issue is that 5th gear is an overdrive gear. on the stock 4.1 diff and current speeds & power levels, I run out of 4th gear at start line of TWS and am at/near redline in 4th in 2 or 3 places at MSRH. In order to still have enough RPM at 1:1, I would need to spend another $1200 to build another diff and run a 3.6 rear end.

OR, I can spend that $1200 difference to get a 5 spd transmission and run the 4.1 and 4.6 diffs that I have currently. I'm not sure which is better/faster in the long run. I really need to sit down and do the calculations, but I need more data than just lap times and end of straight speeds to calculate that- I need corner exit speeds and whatnot, which means I need to borrow a friend's DL1 and run it at each track.

another thing I'm factoring in is that I'm at only ~150whp right now and have no aero. Eventual plans for the car are ~250-300whp and aero, so I'm trying to plan for the future as well. The eventual goal for the car is at least 10 sec/lap faster at MSRH and 15 or so sec at TWS.

Thus, I'm trying to plan for the future in a way that doesn't leave me stranded now. the stock transmission in these cars starts breaking about 200hp, and the one I've got had dying synchros in 2,3,4th gears. It's going to need replacement soon, so I might as well buy a tranny that I can use for the long run.

Long story short, I'm not sure a 4 spd would let me go any faster for any cheaper considering I'd have to invest in yet another differential. the cost difference between a 4 and 5 spd is about the same as buying another diff.

15 ways to skin the cat- just trying to figure out what would be best in the long run.