F150 3.15 axle 10 speed equals 3.55 axle 6 speed

Good analysis, you "get it" when it comes to axle gearing, transmission gearing and engine characteristics and how they relate. Many guys get fixated on the rear axle only and ignore the fact that it's only part of the equation. They probably don't realize that many of the OTR trucks on the highway have rear axle ratios in the 2.70-3.00 range and those work because of lots of transmission ratios being available as well as flat power curves.