Not good at gear ratio, and fuel mileage. 3:21 gears

And tire size

 

Look up gears on google.that will get you started. You have to know your tires trans and rpm, but I think it's a little tall of grar,unless you are running super singles ,if not the fuel mpg isn't going to be great

 

If it's on 22.5 lo pro rubber, 3.21's will be about 1300 rpm at 65. You'll be downshifting at speeds below 65 to climb and pass. Fuel economy should be near optimum. I have no idea what running that low an RPM does for the DD after treatment systems.

 

Just remember that the lower numeric number of the rears, the higher the truck is actually geared.

So for example when you pull out in low gear or back up in Reverse with say 3:90 or 4:11 Rears vs 3:21 rears, the 3:21 Rears may feel like you are in a higher gear. The rears that are geared lower will actually make you feel like you have more power for pulling Hills.

BUT, when you cruise down the highway the 321 Rears will give you a lower RPM at 65 or 70 or whatever speed you cruise at and a faster top speed.

The road ranger website has a road speed calculator that shows what RPM's you will turn with what rear and transmission (direct or Overdrive) and tires.

 

My truck had 3:73 and I changed to 3: 42 expecting a better mileage . I thought that would fit better a 550 cat but mileage is about the same . I even replaced turbo, manifold and flashed to 600hp. Now I think: with the 3:73 I was getting over 6 mpg, after all i changed it still the same, but now is fun to drive...

 

I use this website:
http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

You'll have to look up the final drive ratio for the specific transmission in that truck. Fleet 10 speeds seem to be the standard .73 final. Tire size is also most likely 11R22.5

Figure 1300 RPM you'll be right around 66mph, which is fine for a dd13. They like to run between 1200-1400 when cruising so you'll get decent mpg, but they are dogs in the hills.

 

You'd be hard pressed to find the difference. Think 1 to 2%. And you give up some driveability.

 

The step between 9 and 10 is a lot. It's either 1.00 or 1.34, and at 1.00 you'd be about 50mph at 1300 rpm. If you are doing mostly flat you'll be fine in 10th and I can see you getting 7+ mpg easily.