Wierd Fuel Gauge Readings

[Mariach]

So lately, my past 5-8 tanks, as soon as i'm done filling up, my gauge won't read full. it goes to about 7/8ths, then adjusts itself after 5-10 min of driving. and then that first quarter ends up last me like 80 kms.. since this has been happening, i have been getting about 300-350/ tank.. whereas before ( December) i was getting 400 roughly, and in the summer i could almost squeeze 500. i don't know if the gauge is reading wrong, or if something more sinister is afoot in my engine...