why did they use too make 15 speed trannys then stop what were they made for if the use too makethem

I've been thinking hard on this.
So does the aux unit suck fuel economy? I talked to an owner of a w900a all rebuilt with a 5x4 and he gets 4.5 mpg hauling grain.

 

We had used a 15 speed in the Superliner 500 Mack. I don't know any particular detail about it other than to say when loaded you will be happy to have those gears on steep terrain. Otherwise when empty you started off somewhere before throwing the range selector as you speed up.

It was not a top speed interstate truck like 18 wheelers have in common transmissions etc. What it did enjoy best was off road, loaded with say gravel to the top of the box and you have off roading to do to get to where you need to dump it.

Again I don't know the particulars. As long I have fuel in the tank and a bit of room to deal with gravity etc there is usually a gear that rig will like and that was where you stayed in until the situation changes. It was a good truck for the particular work we put it on, but it's not for kids.

A better transmission to my personal way of driving would be 13 speed with double under. She hits a hill, falls down to torque rpm you lift your foot a moment and floor it That truck will down shift to what is half a gear automatically. When that drains off and more hill to run, you down shifted and repeated the half gear situation. I am way better in a slow truck than I am in a fast truck OTR. That load will get there when it gets there.

We had another dump truck, a single axle 5 ton a bit small. For getting into tiny driveways that are being built tight places. (Thje "big" Mack turned into a tanker and trailer puller for those small jobs. Put in 300 gallons fill both tanks hitch beaver to the pintle hook and stay on the job a week feeding that paver, rollers etc.) That truck has a splitter for the rear axle at low and high. Every gear you took you also shifted the axle. So it kinds of gets busy in there. Some nights I dream of a monster 18 speed with two axles to shift and a overdrive that would not quit untilll

61 mph governor. HA... /snarky.

 

It would depend on what engine and rears.
With a 5 and 4 he woula have had at least one over drive and could of been double over, so good enough there. A b model set up right gets decent mileage up to 6 or even 6.5, and so would a good cummins, but that A would have come with an A model cat if it had a cat, and they were not much for mileage, good enough engines, just never was known for any fuel mileage. Some of those puppies had some low rears too though, which didn't help either.

 

It was a 350 big cam. Fresh rebuild. Done by owner, who was a master CAT tech for 6 years.

 

I do not know why he would have rebuilt a 350 big cam and left it a 350, that don't even make sense, but they are known to get better mileage than that if tuned up right.

 

It was turned up a bit but not crazy. 400 or 450.

 

Historically speaking, when the 13 speed had a single OD, the 15 speed was favoured by off roaders, heavy haulers, mountain truckers and go fast truckers pulling reefers and hauling cows.

With the right engine and gear set up they were the cat's meow.
(lots of Cats had 15's behind them)
Faster on the top end than a 13 spd, .78 OD compared to .83 for the 13 spd, also quicker, smoother and more precise shifting than a 13 spd.
The gears were more evenly spaced than a 13 spd, about 400 to 450 rpm between all gears, depending on your diff ratio, where the 13 spd had a big jump from 4th to 5th gear and the air splits in the last 4 gears slowed your progress up to highway speed and on occasion would fail to shift, usually on a hill when the tranny was slower than your throttle foot, even with the jakes on.

Probably my favourite Roadranger trans of all and the one I think Eaton should have focused on instead of that POS 9/13/18 spd trans.
If they would have made it so you could split the top 5 or even just the top 2 gears, it would have been the greatest Roadranger transmission ever.

 

Pete air-trac and Eaton 40s.

 

A 15 will end in .79 in stead of .74. Its ratios will be a little closer than a typical 10 speed.

 

The reason for the deep reduction 15 speed comes down to one word .....Torque .
Actually the lack of it .

Back in the olden days when Detroit had the 318 Cummins had the 335 power house and Cat had the 1674 none of these had much torque . It seems to me like it was anywhere from 850 to 950 average .

So to make up for lack of torque we used gears and lots of them in some cases like the twin sticks and Mack had the triplex and quadruplex . the rears were mostly 4:10 to 3:70 tops in the early days .

Now days we have 1650 to over 2200 lbs of torque on average and rears from 3:70 to 2:30 something or even higher !

Not as much need for the low end grunt we needed to get rolling in the dark ages in the modern era of trucking .

So we find lots of gears gone the way of the metal dashboards and spring ride seats ........ just memories of us old hands wishing we were as young as our old trucks and as hard as the old Hendrickson walking beam suspension .

At least that's my memories of the 15 speed deep reduction stick against the dash, foot flat on the floor, shutters wide open, smoke a rollin, living the dream, days of old !!!!