Do you mean checking by driving at various constant rpms to check what gas mileage would turn out by numerous tries & errors?

I have been taking #'s down since I bought the truck but I didn't drive it by going at a set rpm on the hwy. I know when I want to go @ constant 60mph, it turns somewhere around 2900-3050 rpm, 75mph: 3300rpm, & I think if I go at 2700-2800rpm, i'm probably going around 50mph, not very fast at all indeed.

So assuming travelling at a constant pace(hwy), gas mileage is dependant on rpm right?
Thanks!

91-22re(408xxxkm & counting with a rebuilt long block)- Flatbed/Camper
/33x10.5BFG-AT/Open 4:88/1.5"BJS/
+2"Shackles/Add-a-Leaf/AirLift/Dual-Batteries