6.9 is right, unless someone changed it out before you bought her. I have the owner's manual for the 2000 IS, page 104; 'Fuel tank capacity, including reserve: 6.87 US gal (26.0 L, 5.72 Imp gal)'. Manual for the 01 Tourer (PO had it) page 78: 'Fuel capacity, including reserve: 5.28 US gal (20 L, 4.40 Imp gal').
My 99 IS goes to reserve between 165 and 185 miles unless I'm running well over 3000RPM or into the wind. At 70 it seems to run forever, around 36-38MPG in normal weather (if I'm behaving myself), city or hiway. Into the wind, at 4000 RPM I was looking for gas at 110 miles. On the other hand I have an oversized rear tire and a slightly smaller than stock front tire (speedo and odo are off the front wheel).
When I sit on a Standard/Tourer (dealership, friend) the first thing I think is 'Wow, tiny tank'.

Fred.