On-board Computer

[PHILIP]

I have just changed my car battery as the old one would no longer hold a charge, then the on-board computer displayed PPPP. Its a thirteen button computer in a 1990 325 sport just in case it is relevant. I contacted my local dealer and they just told me to disconnect it again and leave for one hour and it should come ok. I did this and sure enough it worked but now when i press the 'range' button on the computer it tells me there is twenty miles left in the tank even when the car is more than half full. It has been like this for longer than a week and no matter how much petrol i put in it, it still stays the same. Is there some way to reset the computer or would anyone know anything about it.