I have a 2005 - 700 Sportsman - non EFI model, its a PIG on gas, usually can get 100 miles outta a tank, but in snow, 25 or so it really eats gas when your spinning the rear end in the snow.
Its hard to say, do you hit the throtle hard, do alot of hill climbing in low gear??? All effects your millage, but 70 miles should not be an issue, but since its a HOG, carry an extra 2 gallon can, your sport quad buddys will love ya on long hauls!!!!

I think its a 5.4 gal tank, like 18 miles a gallon, id guess??/??
MudsMyName
kevin