Spark plugs and gas mileage
 

Can bad spark plugs affect MPG? My rx8 is using up WAY too much gas.

plugs, coils, bad compression, bad cat can all affect it

Which is more likely with the rx8?

plugs and coils tend to go out quick on the 8....give us a little service history so we can help figure it out better

Well I don't completely know the full service history because I bought it with 14k miles on it, but since then all I've done is change the oil every 2500 miles and I just replaced my air filter with a K&N air filter. I was thinking it had something to do with the new filter. I heard that unplugging my battery for about an hour and then giving it a few hundred miles would readjust the computer to the new air flow.

the oil on the k&n can coat the MAF, so try cleaning the MAF sensor