[gamfa]

A friend of mine bought his first diesel a few months ago and was asking me some questions about fuel economy that I thought I woukl throw to you folks and get your opinion. I have always had Ford diesels and they have always been standard shift but he has an auto. His truck is a Dodge with the Cummins and is a 4x4, ext cab, 1 ton dually with automatic transmission. He was talking to a fellow that works in the county garage today and he told my friend that if he would keep the overdrive switched off on the automatic except when he was on the interstate, he would get better fuel economy. This really didn't seem to make sense to either of us. I really don't know much about driving economy with an automatic trans. With the Super Duty, if I want economy, I drive off the EGT gauge and the boost guage. When I'm towing, it's the tach and the EGT"s to pick my gears. He has neither a boost or EGT gauge, so I don't know what to tell him. We are farmers and most of our driving is in rolling land below 55 mph unloaded and can load up to as much as 27k when moving cattle.

He could use some guidance from some experienced Cummins folks.

By the way, I don't remember the year of his truck, but he said it was the last year for the 12 valve engine.
Edited by: gamfa

 

[hoot]

Todays automatics with the lockup torque converter get nearly the same mileage as stick. The difference being gear selection ability with the stick allows you to optimize rpms without having an automatic decide for you.

Typically an overdrive switch should not be touched. Kinda like your home heating system. You don't have to shut it down. It won't turn on if you don't need heat and neither will the truck go into overdrive if it didn't call for it.

Most owners manuals will tell you the overdrive lockout is for towing... what they don't tell you is it's to stop gear hunting and help save the drivetrain and brakes. Just like the four position shift lever with the big circle around the "D". You always drive in that position unless you need to keep it out of overdrive while towing heavy on hills.

At 55 mph and 27K I can see where you would lockout overdrive. You're not going that fast so your rpms aren't terrible and you got a massive load so you don't want OD.

Automatics are really getting sophisticated. The Allison is run entirely by computer and varies lockup, engine rpm/shift points and even modifies internal clutch engagement on the fly when in tow haul mode. All of the big three now have the Tow/Haul feature which does a lot more than just lock out overdrive. They also control downhill grade braking while hauling big loads.

Hope this helps. I'm new on the Cummins so I can't be too specific their.

Mike