what could be wrong

on my 2000 gt while driving the speedometer said i was doing fifty but really goig a lot slower tires and wheels are still factory. i've noticed the transmission isn't shiftin right could this be the reason

The transmission could be the problem but I would change the vehicle speed sensor first. This can cause and incorrect reading and can cause the transmission not to shift correctly as it is electronically controlled based on input from the speed sensor.

is it by the front passenger tire