Normal condition, no remedy.
Over the life of a set of tires, they change in height as they wear, which changes a variable on your vehicle speed indicater, and that can't be corrected for, along with some other allowances taken by engineering.
( New tires have 10/32 tread, discard @ 2/32 = 8/32 wear x 2 16/32 , is .5 inch difference in tire diameter)
The mpg computer is way off compared to hand math anyway.
Heck on comparing to consumer GPS, there is a degree of accepted variability.
I think with some custom tuning equipment you can adjust somewhere, but don't quote me on that.
I wouldn't lose any sleep on this, quite normal.