Other Fuel System Tank Mounting problems of the 2013 Nissan Rogue

The date and speed entered are averages, because of multiple events at various times and circumstances. I seldom know how much gas I have because the gauge goes up and down without explanation. Since I don't drive a lot I can just keep track of the amount I put in the tank and fill up when I think I should. Today as an example, my gauge showed 1/2 tank. I sat in a parking lot with the engine running while waiting for a friend to finish an errand. During that 10 minutes, the gas gauge went up by about 1/8 of a tank. This was sitting on level ground in a plaza parking lot, so no slant to the tank to give a false reading. So far I have had no serious issues, but due to the fact that I am on oxygen, I could be in serious trouble if I had to wait a long time for help.