I gained 1 mpg with the muffler and about another 1 with the tunes (both city/hwy), the bed cover will say just made the mpg gain more "solid", but my ram air (self built) cai system actually did the most in city mpg...my current mpg totals are...

City (regular traffic): 16

City (very heavy Traffic): 10-12

Hwy: 20-21 with peaks (ya know, no traffic, wind at your back, etc) up to 23 mpg


Also I am running a Hooker Aerochamber (outflows Flowmaster per air bench testing)
MPG is based upon tank fills.