2014 isx 15 gearing question

[Purplehaze]

So I am trying to figure out how to improve my fuel mileage I have an isx15 de-mandated engine tuned to 485hp and 1750 tq a13 speed transmission 3.42 rears with 22.5lp tires and at cruising speed of 70mph I'm at 1510rpm. My question is do I need to get it brought down some by changing my gears or
is that where I should be. I have thought about putting 3.25 gears in but would like some input before I do. I can't seem to get over 5.8mpg with this setup unless I run at 60mph. Also trying to find someone who knows more about engine tuning than I ever will