I think my knock sensor is shot..

[MaXimaSE]

ok, so I was doing more work on my car this weekend.. I noticed on my last tank of gas I only got 269.5 miles from Full to Gas Light.. Sunoco 91 Octane, estimated that to be about 17MPG

When I first start up in the morning, idle hangs out above 1000RPM in Park. When I put it in "R" , it goes down around 900.. Once It's warm, anytime below 40mph, it'll drop to about 900.. at a full stop It'll rest at 700RPM anything above 40mph the tach wont sink.. was getting a Evap Canister Vent Valve "Open" code, so I took that apart and cleaned it out real good and put it back on.. took it out and got it above 50MPH (like the guy at the shop said to do) and had it there for a bit, no CEL yet. My tire pressure is 35PSI on all 4 16" Fuzion HRi Tires (they say Don't Exceed 40PSI)

Im leaning towards my Knock Sensor being the culprit here, but I ain't to sure considering I read the Knock Sensor is a phantom code, so I won't know it is there. O2 Sensors would throw a code though, correct?

Could my **** gas mileage also be coming from the "SHHHHHHHH" I hear whenever Im on the accelerator.

I need to get a vacuum tester.. and figure out where to test. My exhaust shields were just rewelded to the car, because the welds broke loose.. I know the webbing on the pipe looks real bad.. so Im on a toss up of something under the hood or a exhaust leak..