MY 1969 PETE ON YOUR STEPDECK? SHOT IN THE DARK!!

I'm not sure if it was spicer or not, but that is the common pattern for all the aux. trans I have been around.

 

Legally coming up on 46 years, spent 2 yrs. before I turned 18 greasing, washing,busting tires, mechanic & some bootleg driving back & forth to the sugar mill 5 miles away.... & that shift pattern inthe other post is indeed correct.

 

An 8341D is a deep-under auxiliary model, while the T905-C is a direct-drive main-box. However, the transmissions technically make it a double-over set of boxes. T905C is a 900lb.ft. rated mainbox, just about right for an 8V71.

T905C ratios:

Reverse: 7.12
First: 7.23
Second: 3.92
Third: 2.16
Fourth: 1.18
Fifth: 1.00

8341D ratios:

First: 2.40
Second: 1.22
Third: 1.00
Fourth: 0.81

Because the auxiliary is a deep-under model, you'll only use first gear (deep-under) to get going, and really only that if you were loaded. Once you shift out of deep-under, when you cold shift (split both boxes) you'll only return to 2nd (under), not 1st in the aux.

Here are the compound ratios for each given gear in the shift progression:

1st-Deep-Under: 17.35
1st-Under: 8.82
1st-Direct: 7.23
1st-Over: 5.86

2nd-Deep-Under: 9.41
2nd-Under: 4.78
2nd-Direct: 3.92
2nd-Over: 3.18

3rd-Deep-Under: 5.18
3rd-Under: 2.64
3rd-Direct: 2.16
3rd-Over: 1.75

4th-Deep-Under: 2.83
4th-Under: 1.44
4th-Direct: 1.18
4th-Over: 0.95

5th-Deep-Under: 2.40
5th-Under: 1.22
5th-Direct: 1.00
5th-Over: 0.81

I've highlighted the deep-under ratios in every gear above 1st-Deep-Under to show why you don't return to that gear, as the gears significantly overlap in terms of their ratios. The only time I might use one is if I were mainboxing with a light load, as the shift from 1st-deep-under to 2nd-deep-under is actually a narrower split than 1st-deep-under to 1st-under.

What gets a little funky is the top-end splits, as you have a little flexibility here. My general opinion is to use overdrive gears as little as possible. Walking through the progression you'll hit overdrive in the aux a number of times while in the underdriven mainbox gears. The boxes will be fine, but, walk the throttle gently, it'll save you headache on potential bearing and gear-face wear. Granted, you'll be using it for shows and what not, which is pretty light duty.

If I were driving the truck, once I had reached 4th-direct, I would shift the mainbox into fifth (putting you in 5th-direct, which is your true direct gear), and then shift the auxiliary to overdrive. On the top end, at higher speed, you'll be using closer to peak torque from the engine to keep the truck rolling, especially if you were hauling freight.

The one advantage you would gain from shifting to fourth-over and then fifth-over would be a shorter split into the big hole. It's a give and take, and keep in mind, most of this runs through my mind as if I were using the truck in linehaul service. I'd rather have the narrower split from 4th-direct to 5th-direct because she'll pull better in this shift progression. The shift from 5th-direct to 5th-over is comparable to a 13-speed of the same time, and the truck should pick it up reasonably well.

So, if you're wondering the progression I would use, I'll give you two answers, the first is assuming loaded to 80,000lbs (rather, 73,280 for the time frame of the truck).

Ladder through the gears as the ratios suggest, then split from fourth-direct to fifth-direct, then fifth-over.

If I'm bobtailing around to shows or loaded light, though the truck will likely be able to strictly mainbox (just shifting the main), I still don't like this as it will lug the engine, so, my progression would be to start in 1st-under, then shift to direct, split to 2nd-under, and keep splitting between under and direct in the aux.

As you can see, the ratios technically make the set-up a double-over, however, it is in practicality, a single-over set of boxes, as the low-overdrive is close to 1:1, not to mention, the shift progression I would use skips the low-overdrive ratio. Oxbow is correct in his assessment of a couple close gears on the top-end, however, with a T905C the close split is going to be 4th-over and 5th-direct, these two gears are maybe 50-rpm or so off from one another, then you'll have about a 200-300rpm split into overdrive from either gear.

Regarding my opinion about whether to return to a 5x4 or leave the 13, I'd go back to the 5-speed if you can find one, even if it's not a T905C.

Proper boxes, rather than a 10, 13, or 15 hooked to an aux, are a lot of fun and exceedingly flexible. You'll get plenty of practice behind a two-stroke splitting the boxes.

However, if your aim is drivability, just a nice, easy cruiser? Leave the 13, drop the aux in direct, and just run the 13-speed.