Hello,
I drove my 2005 and my 2007 gas tank empty each one time to check how much fuel gets into an empty tank.
For both FZ6's this was 19 liters. But to get 19 liters in you had to fill the fuel a little bit over the underside of the gas tank opening.
My 2005' FZ6 switched to reserve on average 15,2L.
My 2007' FZ6 switched to reserve on average 14,8L.
Which means you have 3,8 / 4,2L reserve fuel. I think 4L is a good average.
When you drive in "cruising" mode which has something about 5,5L/100km you can still drive 72km on reserve.
Some Conversions: (thx @ google)
Gas tank fits: 19 Liter = 5,01926899 Gallonen
Average. Reserve: 4 Liter = 1,05668821 Gallonen
Average cruising mode: 5,5 L/100km = 42,7662879 miles per gallon
Average range on reserve: 72 kilometers = 44.7387258 miles
This facts naturally depend on gas usage, how full you fill your gas tank and the exact switching-to-reserve-point of your FZ6.
BR Robert