Done this @ 55-60mph.
It was during a road trip to another city (57 miles).
I fueled up the car at my city, a total of 114.9 miles. It accepted 3.05 US Gallons
I think it should have been higher of 1 or 1.5 MPG, because, I did some errands in another city.
I changed a lot of things in that car and I'm happy to see the results. ^^
Edit: Just to precise, the car is 100% stock.