You should never judge your milage off "I get ### on a tank right now" But rather you should write down your milage after topping off. Then when you need fuel again, top it off again and write down your millage. Figure out how many miles you put on the truck and divide that by how many gallons went into the truck. That is then your millage.

I just checked out how far off my odometer is off with the 33's on it. And all I did was take the distance I traveled according to the truck, and the gps. And I divided them. Now when I fill up I simply need to write my milage down and multiply it by 1.13. I only did the test on a 50 mile trip, but I will be testing it multiple times and get a average.