You correct. I think you should get at least 8 mpg even with your gears. I think it still has to do with shift points. If you can get the transmission to shift sooner and utilize that torque I think you can get better mileage.
The other possibility is getting a recalibration for your engine for more horsepower which could change the ratio of horsepower to RPM. However this needs to be looked at closely. I can't remember the curves on the MBE4000 but I do know on a Series 60 that the torque curves and horsepower was identical between 1200-1500 on all the same size liters of engines. It was different when RPM exceeded 1500 that the bigger horsepower would cause a slower drop in the torque as RPM went up. This gave better hill climbing as well as increased mileage as the only fuel needed was to overcome the friction. So with more torque and RPM and when the load was not increased it was easier to pull it.
The rear end gears could be changed make them taller to help too get your RM of 1500 at 70 or 75 also.
The big question is return of dollars... Rear end change say $3000 per differential versus 1or 2 extra miles per $3.00 gallon of fuel. Is it worth it?
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