Rear Gear Selection

[James_Douglas]

Hi All,

 

Sitting here at my desk working on the specifications for the upcoming work on the big heavy Desoto Suburban Sedan. I am working up what rear axle ratio I will use.

 

For the last 10 to 15 years, I have been using either the 4.11 gears or the 3.91 gears. With the "Fast" second gear and the 3.91 the car is great around town. I just use second gear and the OD as long as I am not stuck at a light on a big hill here in San Francisco.

 

I do like the really high first gear in the M6, the 3-speed first is a little weak with the fluid coupling.

 

Attached is a screen shot of my gearing spreadsheet. Take a look and comment. At freeway speed the Borg Warner OD of 0.70 gives me a nice flat land highway RPM. However, to get that with the Powerflite and the Gear Vendors overdrive, I would need to drop down to a 3.55 gear set. The problem is coming out of second gear into direct gear the gearing falls off. I am concerned that the mid-range will be a little to low. Of course the standing stop to that point will improve a lot, I am worried it will bog a bit in traffic. I could go with 3.73 and sacrifice a little highway RPM (MORE) to get a closer "high end" of second gear closer to what I am driving now. But, since we will be traveling around the USA in it, mileage is a concern.

 

I will also have an engine that is producing 10 to 20 more HP, or so I think. That may cancel out and mid-range gearing issues using the 3.55. I may have to have a rear axle made up by Curry, and there may be some gears in between i could use and split the difference.

 

Thoughts?  James.

[greg g]

I may have mentioned this before but I have 700r4 in my Studebaker Transtar pickup.Truck weighs 3400 empty, It connects the 259 v8 to a 3.77 something Ford differential.  It came out of an 87 Caprice taxi.  First gear is really too low for cruising an empty truck. Second gear would be fine except when fully loaded or towing.  Second and third are fine around town and fourth is quite relaxed on the highway.  No tach but seems in the 2300 2500 rpm range.

 

And for some inspiration, for your trip this pic is captioned as coming from Mesa Verde.  Having been up that road in an 85 Dodge conversion van I wouldn't want to be in front of this rig on the way down.  But it looks to be doing the job on the way up. But then the car behind may be a pusher...

Edited January 8, 2020 by greg g