So I finally got the Gen 3 LM7 w/ 4L60E in my '66 C10 tuned and running quite nicely. The problem now is that it's shifting way too early - I have the speedometer / VSS properly calibrated to my gear ratio / tire size, but when driving, it feels like it's bogging down HARD especially at lower gears, and not accelerating properly, so I plugged it into the VCM Scanner so I can see what's going on.
From what I can tell, it is shifting gears, but not at the right speeds. I can physically feel the 1-2 shift happening at close to 5 MPH. And earlier today I was in 4th gear going 25 MPH.
Any idea on where I should look first to fix this?