I tested the speedometer with my friend when he was in another car on the freeway. My car was showing only 45 when we were actually going 65. I check the cable is connected on both ends.
Question: could the speedometer gear be worn out or broken. or something totally different. Anyone ideas?

http://chevelleengineer.home.comcast.net/speedo.htm

Jim linked to some great info but really didn't answer your question and for anyone to do so is going to require more info from you Eric... Start with a few basic's, what year make and model Camaro are we talking about? What transmission is in it? Has the car been modified? Transmission change or rear end gear change maybe? Also what size tires are on the back of the car?

If your car is 100% stock there could be mechanical problems like a slipping clutch or bad speedo cable (year of car and type of transmission should tell if it's a cable or electronic connection). If the car's modified something as simple as a tire size or rear gear change can make the speedo wrong.

Eric, have you changed the rear tire size or the rear gears at all? If you did, you have to change the DRIVEN gear that the speedo cable screws to. Worse case scenario, you'll have to pull the tailshaft and change the DRIVE gear, but I got lucky with just a DRIVEN gear. It will never be 100% exact anymore when you change things because the new gear gets you back within a .whatever MPH, unless you use an electronic speedo.