I bought a flowmaster series 40 delta flow. I bought it because it was small, stronger, welded insteaded of rolled etc. Anyway 1st tank of gas I got 15.5 mpg and next tank I got 16 mpg city driving. Before the muffler change I usually got 13-14mpg.
I haven't realy noticed a power increase but the mpg is a nice surprise.
2001 TJ, Warn HS9500i, tera 3st and ome shocks, 33X12.50 MTR/s, 4.56 gears, arbs front and rear