When your car was built it came with either a 120 MPH or a 160 MPH speed limiter. You received the slower limit if you didn't choose to purchase the optional Z rated tires. Along with the 120 MPH limiter you possibly also have a steel driveshaft. If you upgraded to the Z rated tires you got the 160 MPH limiter and the aluminum driveshaft.
The speed limiter in our cars is controlled by the PCM (engine computer) which will cut the fuel when the car is going over the programmed limit. If you change the rear end gears the PCM will no longer know how fast your car is really going, it will still think the car is going the speed shown on the speedometer and so the real speed at which the speed limiter activates will change. You can see the indicated top speed change by looking at the charts below.
Our speedometer corrector goes between the PCM and the speedometer and corrects the speedometer without telling the PCM that anything has changed. There is nothing my device can do to adjust your top speed, it will only correct your speedometer and odometer readings. Correcting the top speed limiter requires that the PCM be reprogrammed with the correct gear ratio. This can be done with an HPP-3, or with custom tuning.