Better mpg with O/D off or on? - Dodge Diesel

Silver R/T · 11-02-2009, 07:13 PM

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?

hamilton71801 · 11-02-2009, 07:16 PM

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.

Silver R/T · 11-02-2009, 08:58 PM

Quote:

Originally Posted by hamilton71801

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...

jay_d · 11-02-2009, 09:52 PM

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.

infidel · 11-03-2009, 09:45 AM

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.

Silver R/T · 11-03-2009, 07:01 PM

Quote:

Originally Posted by infidel

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.

hamilton71801 · 11-03-2009, 09:17 PM

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.

Dinner · 11-04-2009, 12:55 AM

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).

blackimpala · 11-04-2009, 07:24 AM

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

infidel · 11-04-2009, 10:02 AM

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.

Hodge · 11-04-2009, 03:41 PM

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.

dguru · 11-04-2009, 10:49 PM

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.

11-02-2009, 07:13 PM	#1
Silver R/T Registered User Join Date: Oct 2008 Location: Spokane, WA Posts: 391 My Photo Gallery My Classified Ads	Better mpg with O/D off or on? 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? __________________ 68 Dodge Charger R/T 95 Ford Mustang Cobra SVT 01 Ram 2500 QC LB, auto, K&N air filter.

11-02-2009, 07:16 PM	#2
hamilton71801 Registered User Join Date: May 2005 Location: L.A. (Lower Arkansas) Posts: 1,296 My Photo Gallery My Classified Ads	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. __________________ Mine 2001.5 Dodge Quad Cab 4x4 Auto Cummins HTS Ultimate Duty Tranny, Straight piped, Puke bottle relocation, Stage 3 Nozzles, BD J Hook, Vulcan Big Line/Relocation Kit, Quadzilla XZT+, Lock-up Switch Dipricol Optix trans, pyro, fuel pressure on A pillar, Boost gauge in cubby hole Jordan Ultima 2020 *Hers* 2003 QC HO 4x4 Auto Short Box...stock and stayin that way!!!!! Maxbrake *Ours* Coachman SOA 299TT 1978 Glastron Carlson (115 Merc, Hotfoot, completely restored) '09 Polaris RZR

11-02-2009, 09:52 PM	#4
jay_d Registered User Join Date: Aug 2009 Location: ga Posts: 110 My Photo Gallery My Classified Ads	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. __________________ Bone Stock 2006 2500 qcsb auto 4x4. 325HO truck

11-03-2009, 09:45 AM	#5
infidel Registered User Join Date: Dec 2003 Location: Montana Posts: 11,899 My Photo Gallery My Classified Ads	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. __________________ Bill '95 2500 4x4 auto '95 3500 5 speed flatbed hauler stock for one day

11-04-2009, 12:55 AM	#8
Dinner Registered User Join Date: Jun 2006 Location: Thamesford, Ontario Canada Posts: 1,216 My Photo Gallery My Classified Ads	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). __________________ 2001 Dodge Ram 2500 CTD QC SB 4X4: Goerend Trans with Billet Flex & Billet Input / Airdog 100 / Gauges / ScanGauge II / BHAF / Draw Straw / HTT Exhaust Manifold / 4" Straight to 6" Stack / Smarty & TST PM3 Comp Stack / DDP Stage 3's / HTT 62/71/14.

11-03-2009, 09:17 PM	#7
hamilton71801 Registered User Join Date: May 2005 Location: L.A. (Lower Arkansas) Posts: 1,296 My Photo Gallery My Classified Ads	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.

11-04-2009, 07:24 AM	#9
blackimpala Registered User Join Date: Jun 2007 Posts: 337 My Photo Gallery My Classified Ads	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 __________________ 96 4X4 Auto, Gauges, Full time power windows, headlight relays, By-pass filter, KDP Done, Lukes Link, GM Wheel cylinders, removed fuel heater. I need no stinking chips. For those who fought for it Freedom has a taste the protected will never know.

11-04-2009, 10:02 AM	#10
infidel Registered User Join Date: Dec 2003 Location: Montana Posts: 11,899 My Photo Gallery My Classified Ads	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.

11-04-2009, 03:41 PM	#11
Hodge Registered User Join Date: Apr 2009 Posts: 203 My Photo Gallery My Classified Ads	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. __________________ 1994 Ram 2500 4WD 12v Cummins 47RH auto Romans 1:16- "For I am not ashamed of the Gospel of Christ...)

11-04-2009, 10:49 PM	#12
dguru Registered User Join Date: Mar 2009 Posts: 127 My Photo Gallery My Classified Ads	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.

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