RP 1107A - Computer Application Technology Related To Drivetrain Specifications: Gearing

TMC believes that the "Gear Fast/Run Slow" concept is an excellent way to improve fuel economy. It is most effective when coupled with electronic road speed limiting devices. Before specifying road speed limiting parameters, make sure that the driveability of the truck is adequate (utilizing gear down protection or assuring enough spread between road speed limit and the first possible downshift). Drivers should be instructed about the zero-overrun characteristic of road speed limiting, and that more shifting at lower RPM may be required.

Note that in general simulation programs can not  determine the parasitic loss differences in axle ratios, for instance an overdrive 3.08 ratio vs. direct drive 2.28 ratio (assuming 0.74 overdrive). Transmission losses can be calculated and accounted for between overdrive and direct drive. There for the simulation between a direct drive powertrain and an overdrive powertrain should factor in a 1 - 2 percent advantage for the direct drive simulation.