I put a Borla SI/SO cat-back on my 4.6L Screw at about 11K miles.. I immediately gained 1.8 mpg highway, best mod I've done as far as helping the mpg.

The AF1 intake (added afterwards) neither helped or hurt mpg. Stayed the same.

I got Troyers tunes after both of those mods and my city mpg has gone up by about 1.5 mpg and as far as I can tell the highway has increased just under 1 mpg.

So after all three mods I average about 12.2 city and 19.8 highway.

Not bad for a huge Oprah of a vehicle...