60 mpg (imp) is 4.7 L/100km. I can easily do that if tootling gently through the hills with no head wind, one up, no luggage.
I also tend to keep the revs low.
Under adverse conditions at higher speeds, as bad as 7 L/100km.
Wind and speed are the big factors.
Completely off topic... I'm just back from a holiday at Rawnsley Park in the Flinders Ranges (South Australia, around 400km from home).
Towed an Avan caravan (low profile) behind my Subaru Outback.
Going there travelling at 100km/h with a fierce head wind, got 17.6 L/100km. Worst I've ever had.
Coming home travelling at 90 km/h max, some slow sections due to roadworks, no head wind, 11.2 L/km.
(Without the van, usually get 7-8 L/100km.)
The only other factor for the difference, is fuel quality. But i always get the same for both bike and car, RON91 (standard unleaded).