How many miles on your truck? Like wildbill suggested, maybe your converter is slightly plugged. I'd delete it but that's just my opinion. It's only going to cost you 20-30 bucks to get a delete pipe & try it out. Definitely keep the converter in case you want or have to put it back. My overhead is about 3-4 mpg off from hand calculation and I think most everybody on here will agree. But if you're seeing lesser numbers there than before the exhaust you still got something going on.

 

That may be your problem. My overhead is way off. Mine will read 26+mpg on the highway. In reality, I'm getting around 19.