Triker Mike - welcome to the three world!
Sounds good to me - we have a '95 Gl1500 Motor Trike. I believe they use a Ford rear end. At 65mph indicated on the speedo, but about 62 on the GPS, we are running about 3000rpm.
You might know that 3000 rpm seems to be the "magic number" as far as mpg on a 1500 goes. We get around 35mpg 2-up general riding around if we keep it under 3000rpm, about 64mph. Hit the freeway at 70 - 75, up around 3500rpm or more and the mileage is down to around 28 mpg. Loaded for a trip with all the gear and pulling the full Bunkhouse, I expect 25 - 26 on the freeway. Strangely enough, under 3000rpm on level ground we still get about 33 - 34 mpg loaded up. For economy, and just an all around nicer riding experience, we like to stay off the freeway when possible. (example- going from SW MO to MN on US65, avoiding I-35)