slvrcvc

I have a 2012 Sierra that I have very rarely driven in the past two years or so but I am now planning to start using it again. It has a bit under 21k miles. I changed the oil a few months ago when I last drove it around for a few hours. This past weekend I decided to take it out and knock some spider webs off of it...I noticed two things...I drove straight to the gas station cause I was close to E and even though my fuel gauge now showed full...the fuel range on the message center showed like 74mi left...I drove around for like 2 hours...my fuel gauge now showed close to 3/4 tank and the fuel gauge now read 101mi...? Any suggestions on what the problem there might be? Also, the other thing I noticed is that my exhaust fumes smelled horrible...not a gas or anything but more like something rotten/dead...any ideas?