Mine sounds horrible when it goes into MDS with my 24" Magnaflow. I've run several tanks with and without MDS and the fuel savings is marginal at best. Literally got about .5mpg better city/highway combined with MDS on. I turned MDS off with my Trinity. The truck just runs so much better overall. I still get 14-15mpg city and 19-20 highway.
From what I've read about MDS, the truck works harder while in MDS because its only running on half the engine. So the pcm dumps more fuel into those 4 active cylinders before kicking it back to 8. Whereas if it were on 8 all the time it would've dumped less fuel albeit into 8 cylinders instead of 4. So the fuel savings are most likely nil. I do think MDS would work great in an area where it was completely flat and you could cruise at 65 while in MDS. But for the avg person there's hills, traffic, etc. IMO it just doesn't net the fuel savings to be worth the exhaust drone, decreased responsiveness, or the fact that half the lifters are getting very little oil and are essentially collapsing on cue.
