7 day 800 mile ATV Trip with 850SP. How much gas to bring?

I will be crossing the Gaspe Region of Quebec on a week long ATV trip in June. It’s a rugged 800 mile trip that will require us to travel up to 250 km (150 miles) a day.

I just bout an 850 SP a few weeks ago and haven’t had a chance to run it so I don’t know how much fuel it uses. I’m wondering how far someone could go on an average day of trail riding. We will be averaging about 65-80 km/h (40-50 mph). Just wondering how much extra fuel I should plan to take.