Running gear for flat/step

How fast do you like to run, and what terrain.

2 additional information points to consider.

 

I think 3:55 with 13 speed with final drive gear @ 0.73 will run you about 1450 rpm's at 65 mph. I would say if you want to drive over 65 mph alot you should go for the 3:36

 

Dont you need the tire size as well in order to make a call like that?

 

You're right. Just thinking of my truck that had 13sp 0.73 final with low-pro 22.5's. It ran 65 @ 1450 rpm. Yes. Tire size would change that, good point.

 

Fwiw, I've got a 550, 13 SPD, 3.42 rears on 11r22.5 tires, at 70mph I'm turning 1450 rpms. Pulling tanker on the hills, it has plenty of power, but if you get a heavy foot it will kill you in fuel costs.

 

To make this recommendation we need engine type (to know where the sweet spot on rpm is), tire size, expected cruise and max cruise. Trans type (double over or single over 13?).

Cat like 200 rpm less then detroit so thats a factor.


Check out this link from eaton. Very helpful for playing around with rear ends. Idealy you spec the rear so your in the low rpm range of the engine at 65 so you can go faster if you need and drop a gear to go slower. You could also spec to cruse in dirrect.

I have 3.55's with a double over 13 pulling flats with a detroit s60. Its a pretty good set up although, i'd prefer 2.64 so i could run in 11th which is direct and thus less power loss when cruising. The whole idea that a bigger number ratio gives you more power is not entirely true. it does give you more power in your first gear off the line, but after that, being in one lower gear with a lower number ratio is the same as a gear higher in a bigger number ratio. The whole idea of needing 4.XX ratios is crazy since that will only help you off the line and will limit your max speed unless you really wind it out.

 

once you go over 65, your fuel consumption gets so high that you have diminishing returns. Unless your revenue is crazy high its generally not smart to run over 65. Next time you see a guy running 75, ask him at the truck stop if he even knows what he spend on fuel per mile on average. He probably has n idea. In my truck, it cost .35 more per mile to run at 75 instead of 65. .35 a mile is a lot of money