I get around 30 (less on the interstate) on both my GoldWing/GTL and V Strom/Ural. Neither bike is a mileage queen solo, however. As near as I can tell, the sidecar reduces mileage by 20-25% in local riding conditions, and maybe more than that running high speeds, which might make sense due to the significant drag of the car. 40 mpg on either a 1000 Strom or a Wing are in the ballpark (without a sidecar).
2015 R1200GS (W, LC, Waterboxer, Wethead, ??)
2015 Vespa 300 ABS