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Sep 17, 2007 | 03:46 PM

F6Guy

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Help deciding

I now have two three speed auxillary tranmissions to use, I just don't know which to use. one is a brown lipe with a .86 OD, the other is twice as big(spicer 5831-D) and has a .74 OD. I guess my real concern is will the little flathead be able to pull the truck to 70mph through a .74 OD? any input would be great. the rear is a 5.83:8.11 two speed. I did the calculations and I could run in the 8.11 range and have less than 100 rpm difference at 50mphin OD than running in 5.83 range in 4th gear direct drive. At 70 mph I would be looking at around 2800 RPM, I pushing a little over 3000 rpm to keep it above 55mph now.

Sep 17, 2007 | 04:49 PM

52'F-3

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From: south-central PA

Hmmm, i'm sure one will work be better than the other........
i assume your staying with the factory size.

i personally don't see me driving that truck at 70mph very much, or for a very long time.
so i would want more (lower) gear for stuff like hauling weight and going up hills (i guess you wouldn't use OD for that anyway)
what are the other gear ratios in the trannies, is one easier to take off with or have smaller gaps between gears?

all that said, i always like to upgrade my drivetrains...... so i'd use the bigger (larger in size) tranny just because your choices are pretty close and it's more heavy duty.

Sep 17, 2007 | 06:19 PM

F6Guy

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all I have to do is put the auxillary in direct and have my stock ratios. I do not plan on using the under drive as I don't need it with the split rear axle ratio. I am using the stock T-98 4spd with the auxillary. I would not really want to do 70mph either but my RPM would be that much less at 55.

Sep 17, 2007 | 07:14 PM

HT32BSX115

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From: Upper Left Coast

My plan is to use an OD that gives me a 0.71 ratio....The E4OD I have gives me that ratio.

The question to ask is what ratio do you have in the rear axle?

I calculate with the 5.83 rear axle and 0.71 ratio OD I'll get about 2100 RPM at 60mph.

Sep 17, 2007 | 07:28 PM

F6Guy

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Right but will a flathead pull the truck with that much OD? I know I will not have problems with the .86 OD but that will put me at higher RPMs at cruising speed than the .74, of course anything is better than Stock. In my first post you will note that I am using the 5.83:8.11 split rear.

Sep 17, 2007 | 09:17 PM

56f600

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From: Joyce, WA

My guess is the flathead will pull the .74 to 1 overdrive. With a load on the truck or uphill will be a different story. But, with 4 speed main, 3 speed auxillary and a split rearend, your choices to gear down will be plentiful. My '56 with a 360 and t-18 also does 3000 at 55 to 60 with the two speed in high.

Sep 17, 2007 | 10:21 PM

HT32BSX115

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From: Upper Left Coast

Yeah I saw the ratio but I don't get an edit button anymore so I couldn't edit the reply!

You'll probably have plenty of torque on the flat (empty) in OD.

Well, that's cool......I got all the buttons now!