Is this a wierd idea.
 

I have been driving now for about 5 years and this idea has be percolating in the back of my mind for a good while.
I would like your thought's on it.. here go's nothing..
I haul a regular 53 ft dry freight trailer. I don't get in the dirt much and am usually on highway's 99% of the time.
ok the speed limit's on the highway's varies any ware from 55 to 85 and depending on how my truck was setup the gearing would be ok at some speed's and not others. this has always bugged me.

What if I used a really high gear rear end something like 2.75 ratio, with a 13 speed tranny and an aux 4 or 5 speed tranny on the end of the 13.
the 5speed aux tranny I would use to adjust the final gear ratio.
so I slow states like ca, ohio and illanoies I would put it in the lower gears to make the truck sit in my sweet spot at 55 mph. When I get to a faster state I put the aux tranny in to a higher gear so I can maintain the engine rpm in the sweet spot at say 65mph and so on all the way up to 85mph.
I would want the aux tranny to have gears that only change say 100-200 rpm per gear.
I don't know much aux tranny's other then I have seen them used on dump trucks.
this idea is not about getting the best fuel mileage but it's about letting my truck go the legal speed limit anywhere I happen to be with out over or under revving my engine.

thank you for your time and consideration, your thought and opinion's will be muchly appreciated

It's not "weird" at all, what you're proposing to do has been done many times in the past.

That said, with todays high horsepower high torque engines, the trick is to find an auxiliary that can handle the power without going ballistic at an inopportune time.

Also, be advised that when you run a really fast rear axle ratio like the 2.75:1 that you mentioned, that means that your driveshafts & U joints will be under a lot more torsional stress, especially when you have the auxiliary and the main transmission both in their lowest gears simultaneously.

Here's a link to a modern auxiliary transmission that has ratios ranging from 2.37:1 to a .81 overdrive.

It's a twin countershaft design, so it should be plenty strong. Specs say that it can handle up to 17,500 lbs. ft of torque, depending on the model.

TTC: Medium - and Heavy-Duty Transmissions

Using a setup like that would enable you to keep your current rear axle ratio, and still gear up or down as conditions require.

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hey thx for the link!!

so you think that the rear end gear is to high ? maybe 3.0 or 3.25 ?
I need a good gear head to help me figure out possible gear ratio's, any volenteer's ?

I had some time to do some calculating regarding this situation.

I did these calculations assuming a tire diameter of 42" (11R22.5).

Your 13 speed has a hi gear ratio of .73 :1, so at 1300 RPM with 3.55 rear axle ratio, the truck will be going 62.7 MPH.

Change the rear axle ratio to 2.75:1 and the truck will go 81 MPH at 1300 RPM.

Now we'll get to the fun part:

Install that auxiliary trans described in that link I posted and here's what it works out to:

At 1300 RPM with the 13 speed in hi gear (.73:1), the aux in hi gear (.81:1) and the 3.55 rear axle ratio the truck will do 76.3 MPH.

At 1300 RPM with the 13 speed in hi gear, the aux in hi gear and a 2.75:1 rear axle ratio the truck will do 98.5 MPH.:smokin:

Remember, these road speeds are all figured at a leisurely 1300 RPM.

You want to do heavy haul? Just stick the 13 speed in lo (12.31:1), put the auxiliary in 1st (2.37:1), and with the 3.55 rear axle ratio you'll have an overall reduction of 103.57:1!!!! With the engine screaming at 2100 RPM that equates to 2.5 MPH.

You'd have to exercise caution when both transmissions are in low simultaneously, as the driveshafts, U joints and rear diffs would bust up pretty easy with that kind of reduction being passed along through them.

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You can also spec a fast rear end and then just run in a lower gear for slower states. I do this with my 3.42 rears. When I have to run back roads or keep it at 55 for high enforcement areas, I just run it in 12th instead of 13th. I usually don't run above 65 most of the time, so in 13 I am at 1400 with LP22.5 rubber. In 12th, I am at 1400 going 55 mph. My ISX seems to like that 1400 rpm. No issues with running in a lower gear and darn sight cheaper. With faster ratios than my 3.42, just run a lower gear for the speed you want. I know of an owner at my company that is running a 13 spd with 2.64 rears. He just runs in lower gears at lower speeds to keep rpms in sweet spot. There is nothing that says you have to run in 13 all the time. Unless you are pulling heavy on soft ground a lot, there is no need to spec anything special. Granny low in a 13 even with a 2.64 will get you started quite well in just about all but the most extreme conditions. Heck, go to an 18 spd and you would cover all the bases. That would be cheaper than running a tranny and auxillary and having to redo the entire drive assembly.

The low gear ratio in a 13 speed is 12.31:1. Multiply that by that 2.64:1 rear axle ratio and the overall reduction is 32.5:1. A good rule of thumb for "startability" with heavy loads / soft ground is a minimum of 60.0 overall reduction (lowest gear in trans multiplied by rear axle ratio). Less than that requires slipping the clutch / abusing the driveline in order to get moving.

Actually, an 18 speed has the same ratio in low gear, and the same ratio in hi gear as a 13 speed. The only difference is that you can split every gear while you're in low range as well as in high range, so there'd be nothing gained by doing that.

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Why not just go with a 5 and 4, that way you will eliminate the strain on the back box of the 13 speed when running in 12th or 13th overdrive. That way you will have no more the a 200 rpm drop between gears, depending on the the rear axle ratio.

You can play with the gears all you want, but until you get someone to make some changes in your ECM you won't make any big gains.

Not a weird idea, but probably not the best way to go about things. Most truck tires are rated for 75 mph anyways.

A truck specced for 1400 rpm at 70 mph with a 13 speed would do 60 mph at 1400 rpms in 12th. There's more then enough range there to get within the sweet spot.

Besides, speed has more to do with MPG then the "sweet spot". I've done a whole ton of testing on it. Even a 2 mph gain makes more of a difference then 300-400 extra rpms on the engine.

That brings up another good point:

If installing an auxiliary transmission, the VSS (Vehicle Speed Sensor) would have to be on the output shaft of the auxiliary transmission.

If the VSS is left in it's present location on the output shaft of the 13 speed, then the only time the speedometer would be accurate is when the auxiliary trans is in 3rd (direct drive 1.00:1 ratio).

Other than that I don't see where making changes to the ECM would be necessary.

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