One can perform some coastdowns on the highway (time to go from 85 to 75, 80 to 70, etc), weigh your vehicle and get a rough approximation of the work necessary to go a certain speed. do this over a number of speed ranges and then calculate the required HP and regress is against speed. Then the next issue is getting the Brake Specific Fuel Consumption (lb/hr-hr in the US) table for your engine and find your engine operating speed and torque(or manifold pressure, throttle position etc) for each speed and gear and axle ratios and one can get good estimates for your FE.
Turns out that DI turbo Diesels have the best BFSC numbers and along with the higher energy content of Diesel fuel ( and the fact that Diesel "part throttle" BFSC is not that far from the BFSC near WOT) get some amazing FE. Don't forget that with a manual it's also a lot easier to coast. Coming down I-70 from the tunnel to Denver can be coasted at 60 to 80 most of the way.
The power absorbed by rolling resistance will be proportional to velocity, but the force it produces IS (roughly speaking) constant as a function of speed. Similarly, the power consumed by by aero drag is proportional to the cube of the vehicle's velocity, but the retarding force is proportional to the square of the velocity.