I find they are 'good' on gas, except they are quite sensitive to load.
I get about an extra 100ks if the majority of driving is done with just me in the car, you can tell the differance in the mileage the moment someone else is in the car.
Just recently myself, my mate and two pcs plus monitors went from paekak hill to whitby to the portlan hotel back out to paekak hill and used almost half a tank.
This is in a series 7 bat.
I average around 500-550kms on a tank btw, usually filling about 65litres in it. I dont really thrash it though, too much of a noob driver to really throw it round, and its too damn expensive to crash! I AM with NAC afterall...