Diesel conversion

[Lakota_Traveler]

Hello All,
Don here and I have a question or two. Does anyone drive a 20K in weight, and 19.5 wheels, diesel power and a 4 speed automatic with no OD.

I am trying to figure a ring and pinion change. I have 3:21 gears and it is too high. I did a gas to diesel swap and the gears are my last problem. Any help would be appreciated and thanks in advance, Don

[MEXICOWANDERER]

This sounds like a bigger rig. I helped a friend put a Brownie in with his Allison and it got him out of pretty dicey spots.

[bluebirdPT36]

ScottG wrote:
I've never seen a 4 speed auto that wasn't an OD. Sounds like you need to change the gearing to a more conventional 3.73 or 4.1.

The older heavy and medium duty Allison's (HT 740 and MT 654) were not overdrive.
I have the MT 654. It is a five speed. The torque converter locks about half-way through second and stays locked in third, fourth, and fifth. Nothing like your average car trans.

[John_Wayne]

Go to Randy's ring and pinion then go to his site calculator and input the info.

[ol_Bombero-JC]

Lakota Traveler wrote:
Hello All,
Don here and I have a question or two. Does anyone drive a 20K in weight, and 19.5 wheels, diesel power and a 4 speed automatic with no OD.

I am trying to figure a ring and pinion change. I have 3:21 gears and it is too high. I did a gas to diesel swap and the gears are my last problem. Any help would be appreciated and thanks in advance, Don

More info would help (a lot).

(Besides the 20K weight) - Engine is in what type of vehicle?
Engine type and size. (specs).
Trans type?

~

[liborko]

For a best fuel economy you should be cruising at max torque engine RPM. The rest is just math.

[robatthelake]

Perhaps a Gear Vendors O/U unit would solve Your problem!

[randallb]

Determine your sweet spot RPM and then choose your preferred cruise speed. With these two parameters, the circumference of your tires and your high gear trans ratio you can figure the gear ratio you would prefer. Then you need to look at what is available for your differential and choose from what is closest to what you want.
Using a 245/80x19.5 tire, a 1:1 high gear, 1700 RPM and a preferred cruise speed of 60MPH you would want a 2.95:1 rear gear. What your acceleration would be like needs to be run on a simulator.
Randy

[69_Avion]

I wouldn't go too low of a gear ratio unless you are going to be pulling some serious loads. Remember, diesels run well at lower RPM than the gasoline engines.

[ScottG]

I've never seen a 4 speed auto that wasn't an OD. Sounds like you need to change the gearing to a more conventional 3.73 or 4.1.

[answerswillvary]

Moved from Roads and Routes to Tech Issues.