Mileage is stored in the black POLLAK box that is the control unit for the awesome gauges that the GTs are so blessed with. But, if the speed sensor is bypassed or INOP, the control unit is not receiving any info to instruct it to increase the mileage. So, answer to your question, YES..., mileage could be tampered with. If it were a super low mile car, it would be easier to detect if mileage was inaccurate, but with it being a 9k mile car, it will be hard to determine. If it hasn't been tracked or raced, a very experienced GT mechanic could tell by inspecting brake wear, and overall condition, but still gonna be a tough thing to prove, especially if all carfax and Autocheck data is consistent.