I have a Brown Davis aux tank (cheers Caver ;-) ). The fuel filler clicked off at 107.5L. It was easy enough to get 110L in, might have trickled a little more in had I tried. So that's 192L of fuel, call it 185 usuable. Enough for a few trips to the shops ;-) Or if we use Jamo's figure of 89.9 (assuming he wasn't confused with an FM station :D ), close enough to 200L and not a jerrycan in sight.
I do like long-range tanks on trips and not having to bother with refueling queues at the end of a day's drive when you should be kicking back with a cold one instead.

