I have 80 series 16s on a 69 292 1bbl in my 59 1 ton and on a trip to Oklahoma and back I got around 17 mpg and that with 4.10 gears. Went around 50-55 the whole time - didn't push it. And that was with a load in the truck on the way back. (i was surprised too)

My speeedo's broke, and my tach doesn't work, but I did have a GPS that should prove very accurate. That's how I knew speed and distance.

\'59 Chevy 1-ton
'72 c-10 (in progress)
The Great Rust(less) Mountain
\'47 Jeep cj2a