A lot of cars and trucks already have a speed governor. It is usually set to the speed limit of the the tires. Although some, Vans for example, may be set lower.
I would think that it is programed into the car's computer. Whether you could re-program it would depend on how the manufacturer programed it. In multiple option vehicles, I would suspect it could be re-programed. I doubt that dealers would do it, due to liability, and the software is normally very expensive, if available at all.
It is usually set very high, around 112 mph for cars. I know some Vans are set at 89 mph.