I just changed all my plugs except for the #3 because I didn't have the tools to get to it. (I plan on getting the tools friday) I noticed that I was getting really bad gas miles and when I checked it i went from 16 MPG to 11.5 MPG. Could that one plug make that much of a diference? Any suggestions? I used bosh platnum 2 they didn't need to be gaped

 

what did you set the gap at for the plugs?

 

Plugs shouldn't make your mileage drop even if you left one old plug in. I would check over all of your work. As well as check the gap of the plugs.