My engine has a data plate stating it uses x amount of fuel to make x amount of power at x RPM. I can use that to derive the efficiency but then all other calculations I use, such as finding the HP at a given load using the fuel amount I get with MPG at a given speed, are all derived from that ONE efficiency. I know that is completely wrong but I have no other way to do it given the needed variables. My question is, is it possible to derive an efficiency % using just RPM/MPH/MPG/BTU content...? I have a general idea of the truck weight (6500lb) but the more I think about it the more I think all you would need is BTU/MPG/and maybe speed. It seems energy used (in BTU content x gallons burned) to go a certain distance would tell you work done. But I have no clue. What do you guys think?