wul putting a larger diameter drive shaft create enough excess rotation to stop my "50mph @ 6k rpm in third gear" issue?

 

i highly doubt that. check out your original thread i left you a response. all honesty you need to get this tranny issue corrected in a conventional way. you could always run a tire with a huge diameter, but thats just silly.

 

The diameter of the driveshaft will have no effect on the RPMs or top speed of your car. Something is wrong with the numbers you state. I found an online calculator that shows you'd only turn 3,000 rpm at 50 mph in 3rd gear with 4.88 gears (1:1 final drive). If we assume you have 4.88 (drag) gears and your tranny won't shift out of second, you'd turn about 4,500 rpm at 50 mph. Thay transmission cannot possibly be in 3rd gear based on what you state.