Using compression, however it is done, in an automobile or any other noisy environment, just adds a lot of extraneous background. The use of a noise-cancelling microphone, notwithstanding. Some unknowing amateurs even resort to microphone modifications in the quest for more punch in their signals. The best way to assure a good-quality transmission is to properly use the microphone which came with the radio.
Here's a very good article worth reading: http://seattle.eham.net/articles/19996