When trying to figure out your mpg, you should always start with a full tank of gas. Zero out the trip odometer, then check it when you go to fill up again. Always fill the tank. So when you fill your tank you will know how many miles you drove and how many gallons of gas you used to drive those miles.
We all have the same size gas tank, I get about 260 miles from a tank of gas. "A tank" to me is usually filling up at below 1/4 tank and takes about 19 gallons. My avg mpg is 14, but that is with 35's, a full size rack and my speedo has not been adjusted for the increase in tire size.
Your odometer will be slightly off running smaller tires, but it is a small difference between the 32's and 33's.