My 80 1HTD 31x10.5x15 standard height, roof racks & basket packed, fully loaded with 60lt fridge in the back and towing a very heavy old Swan I can avg 17l/100k. traveling at 95kph, in fourth gear
Running around town and going to work she uses about 9l/100k.
I do run 42psi cold in the tyres, tyre pressure can have a big effect on economy as well as ride comfort. I don't mind having a harder rider.
Of cause if I act like a Richard Cranium and race everything, the old girl can drink a fair bit more