My GS500 gets horible FE after I drop it on its sides, and then have to Uhaul it 150km home due to damage to the drive chain and clutch lever, resulting from slow speed fall. I filled it up after I replaced the brocken clutch lever, and it came out at 4.1L/100km [57 mpg], a number I haven't seen since I was bracking in the new motorcycle. I suspect that some of the fuel leaked out when the bike was on it's side for a minute or two. the section of road had been rained on that day and it was muddy with almost no traction. without traction motorcycles have a hard time standing up.
