Help me pick a gear for my combo

check out wallace racing calculators. Have you punched in your info to see what kind of RPM drop you will get with each ratio? That would be what I would do first chances are the 3.91 wont give you as much of a drop as you are after. EXP: plugging your info in assuming you are around 3500 RPM at 55mph 3.91 will only drop that maybe 500rpm.