Hi there,
Long time reader-first time posting.
I'll give my .02.
I spent tremoudous amoung of hours on dyno and test track doing SAE test and stuff.

What I suggest, do a highway test. Find yourself a portable tank that can be weighted(5gallon is good and a good weightscale) At cruise speed, switch tank and time record for 1/2 hour ride. make the ratio for pound of fuel/hrs. This will provide you with all energy loss while running at cruise speed. Do it a couple of time, to see if it repeat.
Volume is innacurate, weight is much better.

If you have acces to a rolling dyno that measure fuel rate, O2. You could install vacuum gauge, exhaust gas recorder or other.
Spin the dyno at 65 MPH at a steady RPM. Measure fuel rate without load, that will give fuel rate needed for all the vehicule resistance component(tire, axle, trans, engine). Keep it steady at 65 mph and then start to load until it lug (or your assumption of lug, should be at kickdown point).
Make sure you measure increment of fuel rate vs hp during that session(as futur reference).
Example:
without load: 5pounds/hrs
+10HP : 5.1 pounds/hrs
+20HP: 5.2 pounds/hrs, etc.


Deduct no-load-dyno-fuel-rate from highway-fuel-rate. This give you aerodynamic resistance fuel rate and front tire rolling resistance.

Now, since you are still attach to the dyno(caus you did the highway test before going on the dyno), take the highway-fuel-rate, load the dyno to that fuel-rate, keep the fuel rate steady(or dyno load) and reduce dyno rpm until it lug.

Prepare to be suprise how deep it will go. If you go that route (deeper RPM) it will downshift more frequently than before. Other point to consider. if per example, you find that it could do confortably 65mph at 1500RPM, the engine may lug if you go 75mph at 1700RPM, 85mph at 2000rpm. Energy demands is sqareroot of speed.

As far as future number increment test. If you ever do a modification on the car(change tire, synthetic lube, different rearend, manual trans, overdrive, procharger, etc) you will be able to compare the improvement on your next dyno visit.

If you don't have acces to dyno, there are other way(hitch and BIG camper), but poorly accurate. Even dyno result can be questionnable(don't ask).

I did that countless time with class 8 truck. Fuel(gas, nitro, diesel, E85, Hybrid) is energy, regardless of what it does.
Rej