Both the gauges stay to the top when the car is on and only go halfway down when the car is off... Any ideas?

 

Try a hard reset (disconnect the battery minus wire from the battery and touch it to the positive post for a few seconds, then reconnect it).

If that does not work, your instrument cluster has likely seen its last day.

 

Worked thanks

 

Just bought this car today, 1998 XJ8. Left the Dealer and went to fill up and only took 3 gallons and the gauge wouldn't go above 1/4 tank.?.?

 

If your other gauges are working, chances are the sender in the fuel tank needs to be replaced.

 

Quick response....I guess ya have pull the tank?