I have a 99 Expy with @ 15K on a rebuilt 5.4 (only thing reused was the block)
factory pistons, new rods, ect ect only change was new mustang GT heads and cams(heads are PI's like the ones that came off, only diff is the cams) I'm running the GOTs mod and 35" BFG's on stock 17" wheels with 3:73 gears. At 45ish when it shifts into overdrive it runs around 19-21 mpg on the flat with cruise on, but I hear a slight spark knock under light acceleration. I have new fuel pump, strainer, filter, plugs, COP's and O2 sensors, knock sensor, cam sensor, flowmaster muffler.

I'm wondering if a tuner with can tunes will work, or if I need custom tunes to fix this, seeings how I have the GOTs and other stuff can tunes wont exactly be the correct tune for the truck.. thoughts please

 

It's probably the mustang cams....call and talk to Mike at Troyer performance or Mike with 5star.....let them know what you did...

What octane fuel you running now?

You may want to try and go up in octane to see if the knock goes away....

Unlike trucks Mustangs are known to have more power in the upper rpms...
Whereas trucks have their power curve lower in rpms....the 35s ain't helping either...should be running 4.56 gears with them...