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Old 08-15-2018, 10:02 AM   #1
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Need some education about engine/transmission/differential pairings

My brother-in-law sells commercial trucks. He was surprised that my Diesel Pusher had a 4.78 differential ratio. He thought the high ratio was due to using a 6-speed transmission (Allison 3000) paired with an ISB.

I know next to nothing about this but was wondering if there are DP's with greater than 6-speed transmissions. Is there any truth to what my BIL is saying? Of course, his opinion is limited by what I could tell him so maybe I left out a critical piece of information?
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Old 08-15-2018, 10:27 AM   #2
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I don't know if truck automatic transmissions are the same as those in RVs. But your trans is a double overdrive set up , 4th gear is 1-1 and 5th and 6th ODs.
The weight involved is also 1/2 that of a semi , so there will be differences .
My coach with a slightly larger engine has a 4:63 ratio.
Engine torque band , trans ratios, tire size , weight and differential ratio , all play into how a vehicle performs on the highway ; particularly on the hills.
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Old 09-05-2018, 05:24 PM   #3
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Hello Waywards

I would contact your local Allison Distributor and see if they can run what is called an Iscaan. It will tell you once all the info on weight, tires, etc. will work correctly and the speeds you can achieve in each gear.
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Old 09-05-2018, 06:57 PM   #4
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And what ratio would the BIL propose for an equivalent GVW box truck with the same power train? Most salespeople only know what they sell, not why.

Probably most of what he sells use a larger displacement engine with a lower rpm powerband.

The people at Freightliner that design your chassis got it right.
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Old 09-06-2018, 08:06 AM   #5
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Allisonman makes a good point. The Allison iSCAAN program is a great performance predictor that allows a sales person to choose the drivetrain components based on what the vehicle will be doing. Most truck OEMs have a similar program that guides their sales people to choose the correct drivetrain based on the vehicle's expected duty cycle.
Different vocations (motorhome, car hauler, pickup and delivery truck, school bus, etc.) have different performance requirements (gradeability, startability, minimum engine speed on x% grade, RPM at cruise etc.) based on their usage.
The idea is for the customer to tell the vehicle sales person what kind of vehicle they want to buy and what it will be doing. The sales person can use the system to choose the components accordingly to make sure the vehicle that is sold will meet the customers' expectations and the requirements of the duty cycle. A vehicle can be spec'd for fuel economy, performance and everything in between. It's not a situation where all vehicles of a certain type get a specific axle ratio. It depends on tire size, engine, gross weight, and vehicle frontal area (wind resistance). Sure, some components are commonly used but it's not an absolute.

As for a motorhome, usually the chassis manufacturer does all this in advance based on the knowledge of what the vehicle will look like after it gets it body put on.
Sorry to be wordy, but I hope that helps.
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