C 13 and black well 4.65 axle gears .

Yeah not road gears. I hope you have tall 24.5s at least. If not that would bring the rpm's down some.

Do you run heavy or oversize?

 

4.5 at 1900 doesn't seem terrible.

Slow down and lower rpms or try change rears. But I wouldn't expect huge results that will rapidly pay for themselves

 

If it's a good truck it's worth swapping rears. The price of equipment is ####ing insane right now

 

It seems like with the right gears and keeping your foot out of it you should be able to get close to 6.

 

I drove a c13 in a ant eater for a farmer hauling grain in a hopper. could routinely get 7 mpg hauling one way coming back empty.
9 speed and 3.55 gears with tall rubber
Biggest pig ever but I got good mpg and the boss was happy

 

Perhaps I should have said that differently.
Will it make a difference in fuel mileage yes. How much I don't know.
You need to run some numbers on it. Figure out how much it would cost you to switch rears first. Then based on how many miles you run a year and avg fuel price you can come up with a rough guess on return on investment.
If you only run 50,000 miles a year vs 100,000 miles a year that would make a big difference on ROI