Gear Ratio Question

Drive slower. Rpm at 1400 cruising with low boost will help your fuel mileage noticeably.

 

Personally I don't think you'll gain much in terms of efficiency to make a regear worth it. Myself I'd wait until a rear end needs replacement and then regear both.

 

Look up the info on the model number of your rear ends. Various rear end housings can only accept a certain range of gears.
My Merritors with 3.91 can only go down to 3.42

 

I'd be more concerned about keeping it out of boost. If try my best to keep it below 15-20psi on the boost gauge I can do pretty good for a big block CAT. My Q3 mpg was 7.44 driving a 3406e turned up to 750HP, 13 speed, 3.55 rears and about 1500 rpm/70mph.

 

You also have an E model so running 1500 all day doesn’t affect you as much as an Acert where 1500 is the upper end of where you should run. I agree that driving by the boost gauge will help.

 

On flat land, I keep the boost gauge barely registering anything. Just pretend you have an egg under that right foot. Use extremely small movements for acceleration/deceleration.

I would recommend a tune but speaking of such things here is zapreshcheno.