Hi Allison man; I was probably not very clear on what I wanted to do. I currently have a th400 behind a 454 chevy in a 28ft. motorhome with 4:10 gears in the differential. What I want to do is put in 3:21 gears in the differential and install the AT545 that I currently already have. When we compared the gear ratios between the first three gears in both transmissions the allison will still be lower in the first three gears with the 3:21s than the th400 is with the 4:10s. Thus giving me a 20% overdrive when I shift the allison in to 4th gear. See if this makes sence. When allison ran a scan it started falling in the acceptable range when we went with 3:50 gears. But the bottom line is if the first three gears are still going to be lower than what I have now then what would be wrong with using 4th gear as an overdrive.