I went with the Bully Dog because the sales manager at the dealership that I use recommended it. My previous tuner, in my 03, was an Edge Juice. It worked really well, but I did have some stalling problems so I decided to try this one. The Bully Dog does allow you to tune the speedo to your tire size, although I have not checked it to make sure that it really did anything. There is definetely a power difference, although for street driving without a load, I don't really feel that there's a need for the increase. I netted about 1 mpg increase, far below the 3 mpg increase that I realized with my 03 and the Edge product. I would not recommend spending the $500 for that small of an increase. I'll be taking my truck in for it's 10k oil change soon and will talk to the service manager about the milage issue, although I know the answer already that I'll get. It will be "We're not aware of anything wrong"? Always the same answer, until GM sends out a bulletin in about 6 months...