the speedo adjust is to calibrate your speedo with a GPS to make it accurate. fuel gauge is a common problem, they had to fix mine when i first bought the boat. check your paddlewheel on the back of the transom to see why the speedo doesnt work
There should be a purple wire running from the back of the ignition switch and then chained from instrument to instrument. It bring the 12Vdc to each instrument when the ignition is switched on. Make sure it's connected and has 12~13 volts on it.