COMPUTER PROGRAM on 2003 Chevrolet Avalanche 2500

Hi there:

How should i know if the previous owner performed a modification on the computer´s program in order to gain gas mileage or to achive some more power ? He instaled big tires on it,

I´ll aprecite your advise, thank´s

Asked by for the 2003 Chevrolet Avalanche 2500
Have the software version checked and check it with GM. It should match a very specific version, if not then it has been modified.