when it comes on stop at the next petrol station and see how many litres you can squeeze in, your car has a 50l tank. This should give you a rough idea.
I get at least 40 miles in my hgt from first light, not risked further but have never had to put more than 55l into a 60l tank.
I've had 35 out of it, and the needle was just below the utmost minmum marker line on the way back from Newcastle last year. I wouldn't push it much further though!!