Stay under 65 MPH if possible. I used to do worse in HWY riding than stop and go city riding.
On I-95, the limit is 65 MPH and traffic flow is 70-75 and I stopped using it because it just KILLS my fuel economy. By changing my route (taking local roads to the DC beltway and keeping it 60ish), I've increased from 30-35 MPG to 35-40 MPG. In fact, my most recent tank is a new all time high for the Valkyrie, 40.78 mpg (2.54 G/100M) and it was all on my new route.
All I do is coast whenever possible, keep my speed my speed down when possible, tires are 4 PSI (10%) over sidewall max, and I draft behind 18 wheelers whenever possible. I also have a tall, 2-up, windscreen that I use at all times.
Oh, and to echo what everyone else has said, it is the smoothest motorcycle engine I have ever had. I traded my Yamaha Road Star Warrior (v-twin) for the Valkyrie and the flat-6 is sooooooo smooth. The vibration was so bad on the warrior that it would blur my vision on long distance rides because my glasses were all over my face.
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