Duramax/Allison Gearing Question

[Lonnie P]

I'm currently planning a conversion on an '85 Chevy Pickup & have to select differential gears. Engine is a 2005 LLY with Allison. Tire size will be 35-37" radials & I am unsure of the gear to use. Contemplating 3.73's vs 4.10's. Has anyone installed large tires on their vehicle & been disappointed with the performance loss or changed gears & regretted it due to mileage or other issues? Obviously there is considerable expense in a gear change, so I want to buy the correct ones to start with. Any opinions are appreciated.

[tufcj]

Factory gearing on my 2003 (LB7) is 3.73. I'm running a 265 tire which is about 31" tall. With a 35-37" tire, I'd go with at least a 4.10 if you don't plan on pulling anything, I'd even contemplate a 4.56 if you're going to tow.

I know it really doesn't pertain to diesels, but I have a Jeep (about 325 HP) with 37" tires and 4.88 gears, and sometimes they aren't enough on mountain passes.

Bob
tufcj

[Lonnie P]

I'm with you on the gearing issues.
I have another pickup with a built 427 motor, 4.56 gears & 35" tires. The gas motor definitely needs the gears.

I can commonly get rears with 3.73's & 4.10's to save me some money. I was hoping the extra diesel torque would alleviate the need for 4.56's (especially after I turn up the power a little) I'll probably stay with 35" tires as they cause the least amount of fitment problems as well as the other legal issues regarding bumper height etc. Gearing wise, this tire is almost an equivalent with 4.56's as to stock tires with 3.73's.

I hate to gear it too steeply as I would also like decent fuel mileage. (the less fuel it eats the more I'll get to enjoy it) I could easily build another 500hp big block that only gets 10mpg.

I do not have a lot of experience with the Allison models & how/when the converter locks, but I do know the stick shift vehicles seem to fall off power wise once you get too far under 1500rpm. I do not want to have it constantly shifting because it is undergeared.

So, anyone with bigger tires on their truck...... how does it run compared to stock?

Thanks,

[More Power]

I'm currently planning a conversion on an '85 Chevy Pickup & have to select differential gears. Engine is a 2005 LLY with Allison. Tire size will be 35-37" radials & I am unsure of the gear to use. Contemplating 3.73's vs 4.10's. Has anyone installed large tires on their vehicle & been disappointed with the performance loss or changed gears & regretted it due to mileage or other issues? Obviously there is considerable expense in a gear change, so I want to buy the correct ones to start with. Any opinions are appreciated.

If you plan to tow above 8,000-lbs, I'd want the truck geared similarly to the newer trucks with stock tires (245/75R16) and 3.73 differential gears. This tire and gearing combination produces 2000-rpm at 70-mph.

If you aren't planning to tow, gear it to run at 1800-rpm at the highway speeds you run at most of the time. I'm using 3.42 gears in my Lil Red conversion, and love that gearing. I don't tow with this truck. It is a joy to drive at 75-80 mph. With the 5-speed Allison and 275/55R20 tires, it turns 2000-rpm at 80-mph. :)

Jim

[Lonnie P]

I doubt, I'll ever tow much with it, other than possibly my Camaro to the track about 30mi. away.

"More Power" Your gears/tires ratio wise are very similar to 35's w/ 4.10's

I may have a little more weight than you, but not sure of the total as the truck is in pieces (body off, bare frame, no suspension & gutted interior) & the motor will not arrive until this weekend. I have a big project ahead if me.

Any idea of the weight of the engine, trans & xfer case combined?

[More Power]

If I remember correctly, the bill of lading for a crated Duramax, Allison & NVG-263 was about 1600-lbs - for the last one I had shipped.

Jim

[Lonnie P]

Thanks for the info. I made a lucky guess of 1500#. My 5000# truck will gain about 500-600# drivetrain weight going from the big block combo to the Duramax, plus the extra weight of the bigger front axle.

I was reading about your project truck... that is a very nice buildup.

Thanks for all your help.