I bought my 04 Ram 1500 in november 03. Drove it a week, slapped on a Flowmaster, and picked up 2 mpg, putting me at 19-ish on the highway (not bad for a full-size truck!). Drove it until about two years ago, when the weld let off and the V-baffle inside the flowmaster started to rattle. So with 275,000 miles on the clock, I assumed the truck would not last much longer, so I went to the muffler guy and just asked for a cheap glasspack, to get me through until the engine gave up its ghost. The guy STRONGLY encouraged that I use a Magnaflow, instead. Basically, a oval-shaped muffler, built like a glasspack, but with stainless steel "steel wool" inside instead of fiberglass. Also, the outside case is stainless. And the tube through the middle was almost straight.
I slapped that sucker on, hit the highway, and I now get 21 mpgs, and it still has that aggressive tone, but about half as loud.
I will use Magnaflows from now on on everything.
The rest of my setup is stock, with cats, 2-into-1, mufflers were 1-in/1-out with a downturn.
I will turn over 310,000 miles in the next week or so. This hemi is indestructible!
2015 24' Rockwood UltraLite 2304DS behind '14 Diesel Jeep Grand Cherokee/'04 Hemi Ram 1500. Equal-I-Zer 4-Point 1000lb sway control.