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Hi Big Dave
Your experience with the Gearvendor overdrive unit has me considering one for my Ram 3500 4x4, 6sp, 3.73 rear end. However, highway rpm would fall to around 1500 from 1925 with the current stock setup.
Isn't that a bit off peak fuel efficiency for the 5.9 L Cummins, I read that they are most efficient at 1950 rpm but then I don't want to end up driving at 80 mph as wind resistance would kill the potential fuel efficiency gains.
I am a bit surprised at the 15% gain you seem to attribute to the gearvendor that lowers the rpm by 22%. At highway speed, around 40 wheel hp is necessary to move a 7500 lb truck and the rest (a major portion) is to compensate for wind resistance. The overdrive unit will reduce losses due to engine and auxiliairies operation but will do nothing to offset losses due to wind resistance and the 7500 lb of weight. I would have expected no more than 10% gains from a 22% reduction in rpm. What's wrong in my thinking?
Ivan
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