Current World free distance record is 2256.9km. That's without extra tanks. In fact it's without any fuel at all as it was achieved in a glider by Klaus Olhmann in Argentina on 2010-01-12.
Not really. As it is a self-launching sailplane there is some fuel usage at the very beginning of the flight. If you divide that by the total distance flown the mileage he got must be quite staggering though!
Cheers,
DG800