I can get 25 MPG at 125MPH in my Cessna 177RG. I think that's difficult to top. Although, from a practical standpoint, that's slower than the airplane "likes" to fly, so I don't do that often. Engine RPM is too low and it shakes a bit. 17MPG at 165MPH TAS.
My 2006 Diesel Jetta returned 42MPG in mixed driving. Not bad, but not up to the previous generation's lofty 50MPG numbers.
I was able to hyper-mile the Jetta and top 60MPG, but I had to do that on a cross Florida trip, using only back roads, with no AC and keep the speed down to 45mph. I had to let people pass A LOT.
That very trip taught me a lesson. I saved $3 for every additional hour I spent in the seat driving around. I'll never do that again. My time is worth a whole bunch more than $3 per hour.
If you want good mpg, I believe It's better to purchase a Prius-like car, enjoy the natural efficiency, and not waste time going stupidly slow.