ok for bout the past eight months iv been driving around with a four foot exhaust pipe coming straight out of my cat. and that was it that was my exhaust. highway gas mileage was starting to dwindle and the sound of the loud exhasut being right under me while driving was getting to be too much. so i decided to fix it make it side exhaust and just put a forty five and then a straight pipe on that comin out as a side exit. but now i noticed that it seems my highway and even city mileage is still bad somewhat. highway actually seems to be worse but its only with my ac on. could someone trouble shoot this for me? i bought some sealant to seal some leaks i had noticed so idk if that will play a big part. but with having a little more backpressure i assumed it would have made my mileage just a little better versus only having a four foot pipe with hardly no backpressure. any suggestions?