My best ever was the 550k3, did Fort William, across the a9? To the east coast then down to Edinburgh, Glasgow and crosed the border before stopping for fuel. I had myself and a weeks lugage ,fuel can and oil plus tools on board, my mates 250 superdream used up the spare fuel. The rest of that week the 250 superdream my mate was on matched my 550 penny for penny at every fuel stop. The 500/4 sidecar could reguarly return 50mpg, I rode it like a nutcase a fair bit of the time. Motorways on flatter routes gave pretty good mpg, I could do Newtown to Leeds and back only needing to fill up well on the way home, carrying a spare bike collected in Leeds. I think my average was 10mpg better on the 550 than the 500, but how much of that was the refinements of the k3 carbs and engine and how much was the sidecar dragging her back I don't know, trying the k3 engine and carbs never really worked out reliably for some reason, possibly the k3 carbs varnished up in the years between being in my daily rider and going into the sidecar bike.
