when i Pit... what is the green dot that is on my speedometer? how do i change it? It only lets me go 48 miles per hour. and on some other cars they dont have it

its the limiter, you .. need it so you dont go over the speed limit, but once your past the white line you can turn it of by pressing L

To make things a bit clearer, it's the pitlane speedlimiter. All the pitlanes have a limit, and in the proper racing cars, it's diffiult to stay at that speed exactly (you could stay lower, but you would loose time). As the guy above said, when your wheels cross the white line at the end of the pitlane, you can press L.