Dynamat and dynamat imitations are designed to reduce sheet metal vibration and kill noise. If all you want to do is kill road noise there are other things that are cheaper and much lighter that will do it.
Raammat Ensolite
EDesigns TekLite
There are a few others as well. And the cheapest is a kind of synthetic carpet padding. If you search the audio section I think you will find more info about the carpet padding and these other products.
The best would probably be a layer of dynamat/dynamat alternative with a layer or two of Ensolite/TekLite/carpet padding. Do the floor, trunk, doors, and even the pillars and the roof if you want to go crazy.