Part of my good mileage may be due to having a tune-up including cleaning of the throttle body when I got the truck 10,000 miles ago.

I think my Ford-engineer neighbor is right, though: The 3" cat back can't hurt my performance, but probably won't help much either.

 

 

Back pressure alone does not help power but the lack of it "can" upset some things that do effect engine power such as fuel mixture. Some 2 cycle engines are especially sensitive to back pressure. Every Harley rider knows if you put open pipes on the engine it will have very poor power output until you richen the mixture. If you put the baffles back in you have to re-jet it leaner to make it run right.
S Falkner