I have a 1999 Dually 4x4 with manual transmission and 4.10 gears, with a smarty upgrade, and I avg. 15 at 75mph. It varies, if I drive slower it gets better....
Smarty is a plug-in controller that manipulates the ECM signals to get more power and or fuel economy (depending on driving habits).what's a smarty upgrade?
I have an '01 with the same rear end
and get about 12.5 at 75 mph loaded or empty
it is better at slower speeds like 17 at 65 mph