" but primarily fuel mileage for an open road cruiser."
IF fuel consumption is a major factor for you A sidecar outfit isn't where to look.
My Ural which I don't run above 60 mph gets about 28-34 mpgs depending on road, load etc. A buds 200x Electraglide w/ factory chair gets 25 mpgs. Buds Goldwing w/chair gets 18 mpgs. Another bud w/hacked 1200GS gets about 30-32 mpgs. A neighbors 07 Enfield w/cozy car does pretty good at 40 mpgs but it'll only run at a steady 40-45 mph. Just from my talking to folks with sidecars I figure adding a chair to a solo you can expect about 1/4 to 1/3 less mpgs than you got as a solo especially if hwy speeds are your intended use. Building a sidecar outfit for good fuel consumption and hwy use seems counter productive to me. If I were to build an outfit for local use I'd consider an evo Sportster or 650 klr. For hwy flying probably an oilhead GS. In any case adding a chair is going to give you a substantial hit in fuel consumption. PS: above numbers are all on the camel piss government mandated gasoline(E10).