This

 

REALLY 16 and 20 thats not good!!! I was geting about 22 to 23 average before my plenum... I got 25mpg on a road trip before and that was crusing at 70mph to 75mph. NO way i can get that now.

 

Winter gas sucks - normally drops 2-3 MPG just from the winter fuel mix ratios. Happens to me every winter. I go from 19-20 mpg around town and drop to 17-18.

The plenum spacer will not decrease your fuel consumption if you are driving conservatively.

 

I would say it's probably your driving style or you might be running a little rich. You should try resetting your ECU.

 

I thought after the plenum if anything you should be running lean?
How you reset it?

 

Write down your radio presets. Disconnect your battery (neg terminal) for about 10 min. , Reconnect. Start car; reenter your presets, go for a quick drive. Enjoy!

 

Ill try it out.. Is there anything else it can be?