I know we have a 22.5 gallon tank. But if you does not take 22.5 gallons at the time you fill it then you do not use the 22.5. If you drove 200 miles and it took 10 gallons to fill it up, you would divide 200 by 10 and get 20 mpg. The only time the 22.5 comes into play is if it takes 22.5 gallons when you fill it up. You are figuring your fuel mileage wrong. You need to go buy the amount of fuel you put in, not the capacity of the tank. And it sounds like your fuel gauge is wrong. shawn