Best I've done on a tank with the SVT is about 530km of 120-130 km/h of 401 cruising. Took about 48 litres to refill, if I remember.
Note re: defroster making the a/c compressor run--
This goes back to the beginnings of automotive air conditioning. Car makers discovered very early in the game that having the a/c compressor sit completely idle for several months at a stretch had a serious impact on its long-term reliability. To get around it, they hooked things up so the compressor would come on whenever the defroster was used.
Not sure, but running the compressor may also benefit the defrosting process, as the a/c (I think) removes some of the humidity from the air, thus less condensation on der vindows.
