[biketopia]

my 07 classic 2500 hd 6.0 2wd gets 14-16 unladen and about 11-12 when towing with mild weight. If you have a temp gun shoot the cats when it's up to temp, there should be higher temp after the cat if they are functioning. Also you can watch the O2 data to see if it's swinging. The 02's might just be flat wore out at this point too. Did you do a cam or tune or anything when you pulled the heads, go back with the same 317 heads or change em up? Pull the valve covers and double check torque on the rockers to make sure one didn't loosen up, and feel each push rod to make sure you don't have loose one that maybe is bent.