I was wondering, has anybody figured out how much better the 6.5 fuel economy is after installing 4" exhaust? ):h

Probaly not one bit of difference, biggest player on MPG I've found is right foot pressure.

Not much, if any. But I tell 'ya, my 5" SS tip on my 4" system makes me :D ear to ear. The ladies seem to 'preciate it too.

If anything i've lost 5 mpg, there goes the right foot again!

I have a 3" pipe with no cat or muffler. I have noticed a small increase in MPG but putting my foot into it to hear that sound has lowered my MPG. My 4" tip is pointed back just a little and its behind my aluminum mud flaps so Its realy not much louder from the drivers seat than the stock pipe with the muffler. However, every body tells me my truck sounds like a Kenworth with a Cat engine now. I can here the turbo real good now also. It sounds awesome. The only reason I went with the 3" is because nobody had 4" pipe. I didn't want to buy a kit. I could hear the turbo before, but not as much as now. Now for some reason in my 92 GMC with the 6.5 TD, I couldn't here the turbo at all. I kept that truck stock. I do remember the 92 had a different air box.