A lot of variables at play here ... could be as low as 9 ... might be as high as 12. Fill it up ... run it for 250 miles or so and then fill it up again. Divide the miles driven by the # of gallons used. Do that a couple of times, average them out and you'll get a pretty good idea of what you're getting. Oh ... you can add, maybe, 10% to your miles driven to account for bigger tires giving you a speedometer/odometer error ...
unless you've re-geared.

 

I average about 7.5 to 10

 

Does that include the error factor for the 35's ... or have you re-geared?

 

Mine acts weird too. I will fill the truck up, run for about 4 days and on the fifth day the gauge will move about 1/8 to a 1/4 of a tank down. Seems to get stuck a lil bit when the tank is full.