Overdrive Off Default Modification
JK Super Freak
Joined: Oct 2007
Posts: 1,595
Likes: 0
From: The Badlands (near Lafayette, IN)
Are you sure it does not effect fuel mileage? Has anyone figured out the exact rpms for best mileage?
And that brings me to another question I have been wanting to ask all you engineers out there....
What uses more fuel......to lug (torque) up a hill, or to downshift, (or OD off) to keep rpms up to climb the hill? I realize that if you had a vacuum gauge and could monitor it all the time and compare mileage at certain vacuum then you would know for sure. Has anybody done this with their JK?
And that brings me to another question I have been wanting to ask all you engineers out there....
What uses more fuel......to lug (torque) up a hill, or to downshift, (or OD off) to keep rpms up to climb the hill? I realize that if you had a vacuum gauge and could monitor it all the time and compare mileage at certain vacuum then you would know for sure. Has anybody done this with their JK?
My mods have cut max highway MPG to about 19-20. I used get upwards of 22-23 on the open road. The 35-50 MPH range seems to have been unaffected by the mods, which is where having the OD off pays off. I still see 24-25 MPG (indicated) for those portions of a drive. Once over about 55 MPH, not being in OD will start sucking up the fuel. With OD off over 55, the engine gets over 2000 RPM in a hurry, where it seems to have an unquenchable thirst.
I have a hardtop automatic Unlimited with 3.73 gears.
JK Jedi
Joined: Feb 2008
Posts: 7,215
Likes: 10
From: Georgia
My experience has been to gain 1-2 MPG for everything under 50 MPH. The engine lugs too much and/or kicks in and out of OD. When changing gears, the tranny torque converter is in slip mode instead of lockup mode. For slight hills and speed variation, staying out of OD keeps the engine just below 2000 RPM, where it seems to have enough power to stay in the same gear and do so without sucking up the fuel.
My mods have cut max highway MPG to about 19-20. I used get upwards of 22-23 on the open road. The 35-50 MPH range seems to have been unaffected by the mods, which is where having the OD off pays off. I still see 24-25 MPG (indicated) for those portions of a drive. Once over about 55 MPH, not being in OD will start sucking up the fuel. With OD off over 55, the engine gets over 2000 RPM in a hurry, where it seems to have an unquenchable thirst.
I have a hardtop automatic Unlimited with 3.73 gears.
My mods have cut max highway MPG to about 19-20. I used get upwards of 22-23 on the open road. The 35-50 MPH range seems to have been unaffected by the mods, which is where having the OD off pays off. I still see 24-25 MPG (indicated) for those portions of a drive. Once over about 55 MPH, not being in OD will start sucking up the fuel. With OD off over 55, the engine gets over 2000 RPM in a hurry, where it seems to have an unquenchable thirst.
I have a hardtop automatic Unlimited with 3.73 gears.
I think that's probably a good assesment. The engine's power band is a tad higher than the overdrive keeps it. My question though.....and not meaning to take away from the OP's idea, it's great and the writeup is awesome is this...Can't anyone accomplish the same thing by just turning the o/d off getting in the Jeep and turning it back on approaching highway speeds? I know, it's kinda a pain, but it'll do the same thing, right?
JK Freak
Joined: Dec 2007
Posts: 714
Likes: 0
From: Daytona, FL.
Hello All,
I introduced myself last week in the introductory section. I have completed a modification which defaults the JK with auto trans to Overdrive-Off and also installs an alternate (easier to reach) OD- select switch.
I have stock 32" tires with auto trans and 3.73 rear end. I find driveability is best if OD is kept off with speeds up to 55 MPH. RPM does not exceed 2000 by much, and is really where the 3.8 L six wants to be. With OD on, it will tend to shift out of 3rd around 40-45 mph if driving easy. At 45-50 MPH, a moderate hill or acceleration will cause a downshift from OD to 3rd. I find staying in 3rd up to 55 MPH saves shifts, allows the torque converter to stay locked, gives better response, and does not harm fuel mileage. Now others with different gear ratio, tires, etc. may feel differently.
If anyone is interested, I have posted the pdf file detailing the overdrive off default modification. Click on the link below:
http://home.comcast.net/~kingslea1/Jeep/ODMod.pdf
I introduced myself last week in the introductory section. I have completed a modification which defaults the JK with auto trans to Overdrive-Off and also installs an alternate (easier to reach) OD- select switch.
I have stock 32" tires with auto trans and 3.73 rear end. I find driveability is best if OD is kept off with speeds up to 55 MPH. RPM does not exceed 2000 by much, and is really where the 3.8 L six wants to be. With OD on, it will tend to shift out of 3rd around 40-45 mph if driving easy. At 45-50 MPH, a moderate hill or acceleration will cause a downshift from OD to 3rd. I find staying in 3rd up to 55 MPH saves shifts, allows the torque converter to stay locked, gives better response, and does not harm fuel mileage. Now others with different gear ratio, tires, etc. may feel differently.
If anyone is interested, I have posted the pdf file detailing the overdrive off default modification. Click on the link below:
http://home.comcast.net/~kingslea1/Jeep/ODMod.pdf
Thread Starter
JK Enthusiast
Joined: Feb 2009
Posts: 155
Likes: 0
From: Mechanicsville, MD
I have tried driving the same route repetitively for weeks on end with OD and without. The route was a back country route with speeds varying from 40-55 MPH. Some small hills and turns. Without OD disabled, much shifting between 3rd and OD as you might imagine. Not totally scientific but resultant MPG as shown by the computer was 21 with OD and 21.5 with OD disabled. To tell you the truth, to save the shifts on the tranny, and prevent the big drop in RPM, then the downshift, I would accept worse MPG and still disable OD.
One thing to keep in mind, per the service manual, the torque converter does not lock up in 3rd gear unless OD is disabled. This could have some detrimental effect if bouncing between 3rd and 4th gear.
Thread Starter
JK Enthusiast
Joined: Feb 2009
Posts: 155
Likes: 0
From: Mechanicsville, MD
I think that's probably a good assesment. The engine's power band is a tad higher than the overdrive keeps it. My question though.....and not meaning to take away from the OP's idea, it's great and the writeup is awesome is this...Can't anyone accomplish the same thing by just turning the o/d off getting in the Jeep and turning it back on approaching highway speeds? I know, it's kinda a pain, but it'll do the same thing, right?
In short, yes, one can accomplish keeping OD off as needed through manual action in any desired speed range. The reason I wanted OD off by default, is most of my driving is on roads at under 55 MPH. So I want OD off by default then I turn it on if on a road which allows me to drive over 55. In my opinion, when RPM starts to get much over 2000, fuel mileage will suffer if OD is not enabled. Also, I almost never use the factory OD switch anymore, I use the closer switch mounted on the side of the cupholder
JK Enthusiast
Joined: Mar 2007
Posts: 265
Likes: 1
From: Downingtown, PA
Great writeup - thanks. I've been thinking about a similar mod. I was thinking along the lines of using a 555 timer to control a conventional relay, along with some additional glue circuitry to provide the momentary close/open sequence. Your solution is much simpler (though more costly at $106 
). So now I need to decide if I want simple or cheap.....
All that's really needed is a relay wired in parallel with the factory switch, and a timer to wait a few seconds after startup then close and reopen the relay, correct?
). So now I need to decide if I want simple or cheap.....All that's really needed is a relay wired in parallel with the factory switch, and a timer to wait a few seconds after startup then close and reopen the relay, correct?
Thread Starter
JK Enthusiast
Joined: Feb 2009
Posts: 155
Likes: 0
From: Mechanicsville, MD
Great writeup - thanks. I've been thinking about a similar mod. I was thinking along the lines of using a 555 timer to control a conventional relay, along with some additional glue circuitry to provide the momentary close/open sequence. Your solution is much simpler (though more costly at $106 ). So now I need to decide if I want simple or cheap.....
All that's really needed is a relay wired in parallel with the factory switch, and a timer to wait a few seconds after startup then close and reopen the relay, correct?
). So now I need to decide if I want simple or cheap.....All that's really needed is a relay wired in parallel with the factory switch, and a timer to wait a few seconds after startup then close and reopen the relay, correct?
