How do I recalibrate estmated mile display?

[Tplayer]

Driving a 1997 Pontiac Grand Prix GT.
On the dash I have a display which show the approximate miles I can drive with the gas I have in the tank.

Ever since I had a little work done my car which probably involved disconnectin the battery my readings have been way off.

For example, when I fill my tank with gas my estimated miles shown is usually about 400 sometime a little less.

When I first started up my car after service my tank was full but it only idicated I had 100 miles left in my tank.
Slowly it began to rise until it maxed out about about 200 miles.

Since then it's just been way off and has not recalibrated itself when ever I refill the tank.

Does anyone know how to fix this little annoyance?