Fuel gauge jumping around unless in drive
 

I've recently noticed my fuel gauge will start bouncing around and then top out when I take the shift out of drive. It will do this in park as well. I found it does it driving if I popit into neutral too. Back in drive the gauge then works again.
Once I am in drive it seems to be stable but I'm beginning to wonder if it is reading correctly now even when it seems to be working.
What would cause this and how would the gauge know what gear the shift is in?
2000 Chevy Blazer LT.

its just wear on the fuel pump senors, its normal on our vehicles. And in drive the gauge uses an average of recent readings to compute your current level ( because the gas moves all around in the tank, ie: up or down a hill) and in park / neutral it is a realtime reading. And to troubleshoot it if you wish use the search function, there are a plethora of these questions and answers to them all.