I do most of my driving in the city 30-35 speed limit. I was wondering if I should leave O/D off or on as sometimes I do drive 40mph and those rpm's are getting bit high. Would I see better mpg with O/D off or on?

 

Leave it off if your just running 30-35mph. Its not good to lug the engine and its definately lugging at those speeds in OD.

 

truck is unloaded and I'm going on flat roads most of the time, why would it be bad for it since cummins has so much tq down low. I would think it would be fine for it to use low end torque...

 

itll eventually stop it up and itll be weak as crap...plus i know they waste fuel like that, look at the tail pipe the next time you lug it...black smoke galore. detroits like to be lugged around...cummins like to be operated at around 1800-2500 rpm.

 

It's no problem lugging down the Cummins, problem is your tranny won't last long locked in OD at low speed.
Leave it off until you're going at least 55.

 

care to explain how it's bad for trans? I'd like to know what's going on when O/D is working/off and how it affect trans.

 

I believe it has to do with line pressure which operates off rpms. The lower the pressure, the worse it is on the tranny...I think. Someone will chime in with the correct answer.

 

That's correct^^ The lower the RPM, the lower the line pressures for the transmission as well as the TC circuit. When you are putting around, lugging the engine in O/D. There isn't as much line pressure going to the TC to keep it locked up compared to if you waited until 55mph. Torque Converters like high fluid pressures.

For the original question, I believe if you wait until 50-55mph you will net better fuel mileage. Main reason being is you aren't lugging the engine (puts more load on the engine which in turn fuels the engine harder).

 

OK, i don't understand I though that the OD didn't come on until 55MPH If that is the case how will it hurt to not touch the button and let the transmission or computer control itself.

Floyd

 

Mph when OD locks varies between vehicles.
It supposed to be 50 mph but I've seen many lock at 45.
More of a problem than when it locks is when it unlocks.
On my own rig once it's locked it won't unlock until 30 mph or less unless I give it a bunch of throttle.

 

If you are in OD or T/C lockup at such low speeds, your oil pressure will also be much lower due to the low RPMs. I personally feel that it is better for the engine to operate within its torque curve, with the oil pressure up where it should be.

 

If you increase the pressure regulator pressure to at least 75 pounds you don't have to worry. Stock pressure is the reason for failure. A TFOD diesel transgo kit and increased line pressure is the best thing you can do for your tranny. Or if you are made of money you could get a performance valve body.

I have a gauge on my tranny and it stays at 75psi minimum when in drive. I see 90+ when cold. I may increase the pressure more next time I have the pan off. Stock is 45psi. My CL locks at 45mph.