Tuner/tire size/speedometer questions

[PaHumBug]

I recently bought this H2 with a lift and brand new 37 inch tires on it. The speedometer was reading 8 mph faster than I was travelling (according to my Garmin), which makes me think the previous owner had larger tires than what is on there now. So I just bought a Superchips Cortex programmer so I could adjust the tire size. When I plugged it in, it saved my "stock" settings. However I don't think my stock settings were actually stock. I changed the tire size to 37, and the speedometer was still reading 8 mph too fast. So had to reduce the tire size all the way down to 30 inches to get the speedometer to read accurately. Does anyone see a problem with this? Will it hurt my truck? I just did it and haven't done anything but driven up the street to verify the speedometer.

There is a $100 program I can buy from Superchips that you are supposed to use if you failed to return your vehicle to stock and took it to the dealer. Maybe that is what I need but if I have the mileage acurate then maybe I can save my $100??? What do you guys think?
Thanks!