The GV works in my opinion, i can feel it shift to OD at about 45MPH. also that's one of my "i think an issue" is it shifts fairly hard coming out of OD. i email GV and still haven't got a reply. what exactly is a "healthy" RPM for these IDI's? i could just be thinking there's an issue when there isn't? i'm only going by what i've read in some other posts )1600-1800 RPM at cruising speed on the freeway. Thanks again in advance.
1600 RPM is not going to be easy to achieve when you run a C6 slushbox. Even if you could get into a range where the final drive calculates out to that, torque converter slip will add a couple hundred RPM. This isn't as noticeable at lower speeds (say, under 45 MPH) when loading on the drivetrain is less. Having said all of that, here's what the numbers look like:
70MPH
4.10 and 0.78 OD = 2371.6 RPM
3.55 and 0.78 OD = 2053.5 RPM
3.08 and 0.78 OD = 1781.6 RPM
3.08 and
0.712 OD = 1626.3 RPM*
55 MPH
4.10 and 0.78 OD = 1863.4 RPM
3.55 and 0.78 OD = 1613.5 RPM
3.08 and 0.78 OD = 1399.8 RPM
3.08 and
0.712 OD = 1277.8 RPM*
*E4OD overdrive ratio with no slip
As you can see, 55 MPH should already have the RPM below 2000 with your current setup, but torque converter slip appears to be eating up some of that, which is why you observed 2000 RPM. Since the truck is 2wd, you could easily swap to 3.55 ratio for a reasonable cost, since there's no front axle to worry about. You'll have to decide for yourself if the 300 RPM drop @70 would be worth the trouble.
To calculate ring & pinion, divide the big gear by the little gear. Example:
40 teeth / 10 teeth = 4.10:1 Axle ratio
39/11 = 3.55 (Calculates to 3.54545454...)
40/13 = 3.08 (Calculates to 3.07692307...)
If your calculated number comes out to less than 1, you got the numbers reversed. Simple.
Nice truck, BTW! I always liked that style of front bumper.