on my '00 ZR2 I cant figure out why the fuel gage went from half tank down to past the empy point then back up to full while idling! i was afraid i ran out of gas, because it stays at halfway point all the time... any ideas?

 

yup get friendly with your trip calculator.
fill up and set it to zero.
run 200 miles and fill up again.
compare that to the 19 gallons your tank holds
do that 4 or 5 times and average out your fuel econmy.

welcome to owning a Blazer...

if you want to fix this: search the forum. this is probably the most common problem in the blazers and is all over the forum. good luck