I just finished swapping an LSD from a 96 M Model into my base 99. Here is what I think I know. The base 99 open diff is a 4:30 ratio. The LSD from the 96 is a 4:10. Besides settining it up for future use, I think I should have lower RPMs at upper speeds and a little better MPG. Is that true. Here is what I am seeing in 5th gear (only 5 speed)

80mph - 4200 rpm
75mph - 4000
70mph - 3700
65mph - 3500

speedometer spot on as compared to GPS. As I understand these rpms should be 5% less with the 4:10 rear end.

One last question. I assume what I pulled out of the 96 was a LSD Tor II. After I pulled both axles I looked through the axle shaft holes in the diff and there was no bar. My base model diff had the bar as per the pic link

 

RPM at a given road speed with 4.1 diff = (RPM with 4.3 diff) * (4.1/4.3)
Put another way: you'll lower your revs by about 4.7%.

Grade 7 math is fun!

 

Thanks for the confirmation. Was I correct on LSD identification....no bar/shaft as I look through the axle shaft holes