Just had Motive 3.90s installed yesterday, made a 50 mile trip to have it done. On the way over I set the cruise on 75mph for a comparison later and was turning 2000rpms according to the tach needle. After the install I used the predator to calibrate the speedo for 3.90s and did a bit of heat cycling the new gears. Later I set the cruise on 75mph for the trip back home and was turning 2450rpms according to the tach needle.
Looking at the Engine, Gear, & Tire Calculator at
http://www.wcfords.com/stuff/calcs/EGTcalc/egtcalc.php I should be turning 2150rpms with the factory 3.27s at 75mph and 2565rpms with 3.90s which seems to match what I was seeing looking at the tach needle (not the digital tach readout). According to this calculator with 4.10s you should be turning 2696rpms at 75mph, which is a 131rpm difference.
I should add, when I made my gear selection I was not concerned with tenths out at the track. I just wanted to give the car some kick off the line and make it feel a bit lighter. 3.90s have done a good job doing this and I’m very happy with the choice I’ve made.
BTW I haven't changed tire sizes, these speeds and rpms were in 5th gear on 245/45/17s.
-Cale