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

What Transmission ?

 

Your gonna have a big gap between 9 and 10. It will be ok. But won't pull hills the best.

 

Not sure about fuel mileage.

 

The only way to know that really is try it, in theory it should be but you never know.

 

A 3.21 rear isn't going to scream up a hill, but you should be able to get good fuel millage.

As rule, the lower ratio (3.21, 3.36, 2.64, 3.42) the better you'll do fuel economy wise and they're fine for flat land.
The higher ratio (3.73, 3.90, 4.11 +) you'll climb better but "to out" earlier and at highway speeds have a higher rpm. Heavy haulers would fall into this category. They don't need to hail ###, but need help getting up to speed.

I've always heard a 3.55 is about the beast all around rear gearing. But again, it's really going to depend on what the application is.

 

Pretty good all round answer. 3:42 to 3:58 gearing, you can't go too far wrong. Research your gearbox final drive ratio just in case.

 

Take the serial number to the dealer, ask them specifically what 10 speed is in the truck and then walk up to the front sales area and ask them where the sweet spot of the engine is with that trans and rear end. This is where you want the truck to operate at for mileage.