Re: IML: 'overdrive' details/ 3 to six

With a manual transmission, the overdrive unit usually will allow the driver to go into overdrive at a certain speed, such as 35 mph without consideration to what gear the transmission is in. This means that a three speed tranny can become a six speed by adding the option of being in overdrive in each gear creating another set of ratios. My experience has been with my old FMC V-12, which essentially becomes a five speed, instead of six because overdrive is locked out of 1st gear. No big deal since it won't go 35 in first very well anyway. I have not considered adding an external unit to the back of an automatic, although I would think that doing so would have an effect similar to that of a two speed rear axle.

A factory installed automatic overdrive becomes a four speed unit, with overdrive as a "super-high" gear. This is suppose to cut down RPMs by around 20 to 30% from direct drive, resulting in fuel savings at cruising speeds. Acceleration in overdrive is dismal at best since its obvious purpose is to save fuel and engine wear, not increase power. In fact, in overdrive there is a noticeable power decrease due to the inverse ratio, resulting in gradual speed reduction on grades and slopes. Automatic overdrive units are set up to "kick down" into the next lower gear, or further, depending on speed and throttle pressure, in order to achieve the burst of power needed in the case (for example) of passing, or to maintain continuous speeds on grades.

Overdrives were loved by the owners of cars with standard transmissions mainly in the 1940's, '50s and '60s. They were not particularly loved in the form of automatic overdrives due to drivers frequently being annoyed by excessive gear hunting. They are compromises that go in favor of the direction of their intended purpose. The ideal situation for one set of circumstances is compromised in order to better serve the need for success under another different set of circumstances. To ponder making changes such as these to an Imperial only makes sense if the car is being adapted to meet the needs of everyday use, or to be exhibited as an extraordinary custom displaying the creativity and ingenuity of its owner. Most of our cars are driven on a limited basis and mo stly used for pleasure or hobby functions. I have found over the years that it has made more sense for me to own and drive another more modern vehicle to deal with the world today, and save my Imperials as a reminder of the good times they represent from the past. Unfortunately, that policy has made it difficult for me to drive any one of them enough to keep them in tip top condition. Being seldom driven has it disadvantages, but one big plus is that the cars are still here for me to enjoy when I am so inclined.

I would change the gears in the Ram unless you really need them for towing.

A big part of the overdrive units as I recall is that effectively you go from a 3 speed to a 6 speed. This allows you to put higher numerical gears in your rear end (3.55 for example) for acceleration, but still get good highway mileage.

----- Original Message -----
From: "Rob McCall"
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: RE: IML: 'overdrive' details
Date: Wed, 26 Jul 2006 22:42:26 -0400

I haven't tried it yet, but I am heavily considering it for my '04 Diesel
Ram and my '68 Barracuda. In the Ram with 4.10 gears, even in 6th it is
turning the motor too high for sustained highway use. It has more than
enough guts, but the mileage drops like a rock. At 55 MPH and below it's
happy. Over 55 and goodbye $$$.

The GV unit allows gear splitting, i.e. 1st gear/1st gear in overdrive,
2nd/2nd overdrive, etc. With the stick shift in the truck, I can see this
being handy. With an automatic, not as handy.

Now the real question is what rear gear ratio do you have now? If you are
still running the stock gears, I do not think you will see much of a
difference in mileage. A 2.94 rear gear in overdrive is the equivalent of a
2.29 rear gear. If you are planning a run to the salt flats, that might be
a good ratio. If you are running 3.55's, it will drop it down to a 2.77, a
nice highway gear ratio. 3.91's drop down to a still highway friendly 3.05.

I have a spreadsheet at work that I was playing around with during lunch(es)
this week calculating RPMs based on MPH, rear gear ratio, and tire diameter.
If I remember to send it home tomorrow, I'll be glad to share it with anyone
who is interested.

Other than cutting the driveshaft, the transmission tailshaft housing is
replaced with the GV adapter and the overdrive unit bolts to it.

Rob McCall
'67 LeBaron, probably 2.94 gears.

-----Original Message-----
On Behalf Of Damian Justice

yessir, you have to cut, or replace the driveshaft. They have
under/overdrive if you are intrested in a quick takeoff followed by high
speed/low rpm. I am only intrested in low r's on the highway. I guess
nobody has tried it yet.

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