The manual I guess. I have the 5.2 gasser but the 5 speed manual. I was getting the 15/21 before I lifted. I did notice a BIG diff after I went with a K&N and flowmasters. Best proformance mods for the bucks I think. I have a buddy that just put in a "PowerAid" and he told me he had a very noticable power increase. Also said it has improved his milage a bunch. I have him tracking his MPG right now, and he is on his second tank. One I see the hard numbers I may get one for my truck. They are about $80 on the net. I will post my results on here when I hear back from him.
The 15/21 was going off the odomiter too. After I did the lift I installed a GPS so the "after" numbers are very accurate.
Some fun numbers off the top of my head...before the lift and tires: 75MPH=~2150RPM, after the lift, 81MPH=~2000RPM (GPS). I wrote down all of the RPM levels and speeds befor and after if your really interested in seeing them. I'm bogged down now in 5th, but it does fine.