[scarceller]

Be aware that a cyl that is not firing will cause MPG issues if you have the stock O2 sensor connected. Here's why:
A cyl not firing is a O2 pump, the O2 in the air fuel charge goes for the ride straight out the exhaust.
This extra O2 is then mixed with the exhaust from other 6 cyls.
Then the exhaust runs past the O2 sensor and the sensor sees all this extra O2 and thinks lean condition.
But in fact the other 5 cyls are not lean they are burning just fine.
What happens next is the O2 sensor sends this false lean condition to the DME.
Now the DME tries to fix the lean condition by riching the mixture to try and correct the false lean condition.
It's very common to see MPG drop by 20% when this occurs!

Are you certain all 6 cyls are correctly firing?
And do you have the stock O2 sensor plugged in?

You really need to get a WBO2 gauge to properly diagnose what the AFR is at idle, part throttle driving and WOT.