I recently changed my differential fluid and then went to go fill up my whole tank (about 16 gals) at a Newly Built Citgo gas station. Usually I get around 250-285 miles per ful tank. Now im going on 300, and I still have little over a quater in the tank. I havent drove any different. I was wondering does changing your differential fluid improve your mpg or is it the fact that I filled up at a newly built gas staiton? And how ofter should I change my diff. fluid?

Yes, changing the differential fluid will definitely affect your mileage. Change it every 15k if you use dino or 30k if you use synthetic.