Is there going to be a noticeable difference in my gas mileage?

 

Not really.

 

I had 4.77s for years, and really liked them. I have an AP1. Gas mileage stayed the same and I was quicker off the line till about 70-80ish. However I recently installed the stock 4.10 gears again and while the car does feel a tad slower in like 1st gear, I like the low cruising RPMs and the nice long pulls from 3rd and 4th gear.

If I weren't planning on boosting in the near future, I would probably get gears again, however I may look for 4.57s over 4.77s.