If you have a Connected Services subscription or the free trial from OnStar, it tells you the lifetime MPG of the car. My lifetime MPG says 17.4 over 14k miles. I've only had the car since 11k miles and I am sure I'll raise that number.
The previous owner reset the trips when he bought the car at 2k miles from a dealership and he averaged 16.4 MPG.
I also track my gas fills as much as I can, there is a slight discrepancy in the MPG stated in the cluster and calculated MPG.
For my SS, I drove from Florida to Texas but unfortunately didn't reset the trip for that.
SS
v6
I find it interesting I'm getting almost the same MPG as when I had my tuned v6 (v6 with AFM turned off vs v8 with AFM on)