Diesel bike mpg ??

[Nortwestrider]

I'm thinking about building a diesel sidecar rig in the future,my GS850 sidecar rig only gets around 28mpg.
It only had around 50hp new so now probably less.I don't need to be the fadtest thing on the road but would like to be able to do the speed limit.I have a new Honda NC700x that gets around 75mpg on the highway and 63mpg commuting so my sidecar rig doesn't get used much.
What kind of mpg's are people seeing on HEAVY bikes/sidecar rigs ?????.