160 MPH Trans Am Speedometer Upgrade
Ditch your slow 100 MPH speedometer and replace it with a 160 MPH speedometer from a 1974 or older Firebird. or Trans Am. This is a direct replacement. They can usually be found on eBay.com.
Just remove the dash bezel, then the instrument cluster. Now remove the clear plastic with the attaching "pop" rivets.
Now remove the two screws from the back of the speedometer and replace speedometer with the 160 MPH unit.
Here is how it looks in my 1978 T/A: