Living in MI driving about a steady 65-75 I get a solid 20 or 21. This is with the truck in my sig. I have 31" LTXs with a 4.10 rear I turn about 3100rpm at 70mph 2950rpm at 65mph. I think that with these trucks just have a sweat spot with the 5spd that it doesn't have to work so hard when its turning that fast since its in the meat of the powerband. Anyways thats just my observations.

1997 T-100 100k and counting Green
5spd 4x4 Ext Cab isr mod Aisin Manual hubs