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).
2014 Triumph Tiger 800
2015 Vespa 300 ABS
2007 Dyna, 1947 Harley 45 flathead
1975 BMW R75/6