I can do 45mph in first gear and it will downshift into first gear which is something I've had an issue with other automatics.

I think the 2.77 gear ratio would be the only way to go and have a useful 1st gear. The 2.77 final drive has a nice cruising speed of 70 MPH at 2000 RPM (and crazy theoretical top speed). The tall gearing of the 2.77 would also offset the 4.48 first gear. Here are a few analysis screen shots to help explain and visualize.

2.77 final drive:







3.16 final drive:







3.39 final drive:

I agree that the 2.77 final drive would be nice, but it is only available in the 6T70, not the 6T75.

The transmission cases are different enough that you can't just swap them. We have a 6T70 and another 6T75 to see if we can build a beefed up 6T70 with the 2.77 final drive using as many interchangable parts from the 6T75. It is on the to-do list, but I am not sure when that will happen. Right now we are just focused on putting miles on the 6T75 that is in the car.

Those are some very nice graphs! Great software.

I currently cruise at 70 MPH under 2000 RPM (LS4 + 4T65E-HD).

Would it be possible for you to perform some simulations for my vehicle?

255/45-18 using the same or similar constraints (2.77 and 3.16 only please).

Sure... Its free software from performance trends from '98 (trial). Not sure if its still out there or not???

2.77:






3.16:

Thank you for those!

Dan did you get my message?