i mean going one or two sizes down would get you maybe 2 or 3 more MPGs. but that depends on a lot of things too. a good tune up can help as well as properly inflated tires etc etc
THe plugs are fairly new...I could check the air filter..but since I haven't put 6000 miles on it in 5 years...I wouldn't think a tune up would be that necessary...
I guess no muffler may cause extra fuel burning...but it sounds so sweet straight out of the catalytic converter..