I've got a 98 5.2L Ram 4x4 and it's in dire need of a muffler. I currently has no cat, just straight from manifold to muffler. The muffler is completely shot and rusted out. I would like to possibly improve my mileage without making it louder. I'm either looking at a stock replacement muffler or a Flowmaster 40 series for it. If I do the flowmaster is it going to matter what size in/out pipes I go with? Give me some insight to whether it's worth the extra $$ for an aftermarket muffler or save $$ and go with the OEM muffler replacement. I don't want it too loud as I like the windows down in the summer. Thanks!