I bet that question could be answered for a lot of individuals by looking at their fuel logs. For instance, here's mine:
The green trace is when I started hypermiling. I used to average just under 30 MPG, I think, before that. You can see a lot of "noise" (ups and downs), but the general trend upward is quite clear, and I wound up averaging 42 MPG over that span of 4 years. Most tanks in the last two years were 40 MPG or better.