Your gearing is working against any thought of 25 MPG. The struggling at 73 MPH shouldn't happen. As I remember from my 2.6 the Mikuni carburetor is kind of like having a constantly running toilet flowing into your engine. They had a lot of trouble with the choke not wanting to completely open along with internal leakage, thin walls, heat problems, etc. Speaking from experience the Weber (if allowed in your state) will do wonders for your performance mileage wise as well as on accel and top end. I cammed mine when I put the Weber on and was really happy with it up until the day the facing came off the cam and turned into liquid sand paper (Crower cam) and lunched the engine 19,000 miles later. Rebuilt it and was GTG until I sold it.