With my problems with the speedometer and fuel guage, and I also had a performance tune on my gto.
In most cases, the issues with the speedometer and fuel gauge are related; since they receive data and input from similar sources. It could be a problem with an electrical relay from the ECU to the speedometer, a transmission speed sensor issue or perhaps a problem with the gauge itself. However in some rare instances, performance upgrades like the one you indicated you had done on your GTO can cause electrical communication problems from the ECU. You should start by having a professional mechanic complete a check engine light inspection to determine if you've got some trouble codes that indicate where the problem exists.
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