tuner ?
 

The truck in my profile, i was wanting a tuner that i can change my tire size and help improve gas mileage. I dont need a tuner that has monster power, there is 230k miles on the original injectors and the VP and everything else. i just want something to help a little with out breaking anything. Any advice would be appreciated. Thanks

The smarty chip will let you change the tire size.

does it help with gas mileage and HP?

Yes, I think that Smarty is exactly what you are looking for. It does have an actual fuel mileage setting, and even on that is still deffinately noticable with a little more kick.