What is a good Ratio for a 13 speed?

On a CAT it depends on tq less than 1750 ft-lb 1400rpm more than 1750 ft-lb 1325rpm.
If you're running 55 to 65 take a look at 2.64 or 2.79 rears and run in direct or 11th to cut down on gear drag loss in the trans.
I will be going to this setup in a few months converting a tandem/tag axle.

 

Here is a PDF of CATs programing recommendations.
Hope this helps

 

Dude you are right on the money. I currently run a 3.25 and long for a 2.79. I'm looking to combine a rtlo-20918b with this combo and all but forget the upper 2 gears. I'm currently get 7 to 9 mpg with a C15 ACERT and mostly cruise at 55 to 59 mph

Oh, and thx for the link. Great info

 

Wow, how heavy?

 

What model 13 speed should I use for a 12.7 Detroit 550hp with 3.55 gears on low pro tires (255/70)

 

I have 425 cat with a 9 speed 390 gears on 11/24 what would a 13 speed do at top speed I need the pulling power not the top end