That is characteristic of the XP chassis due to the fuel tank configuration. It gets an air pocket in the rear when it is being filled. You don't need to lift and drop the rear of the sled every time you fill up to get a full tank though.To get a truly full tank just fill the tank as you normally would, then when it appears full go to the back of the sled and push down on the rear bumper - you will hear the fuel gurgle in the tank as it burps the air from the rear pocket. When you go back and look at the fuel level in the filler neck you'll see that the level went down when it filled the air pocket. Then you can top it off. It will probably take about another gallon or thereabouts after burping.
Be careful of doing this before storage or bringing the sled into a warm climate since you would want to leave some room in the tank for expansion of the fuel.
