[WSSix]

Road noise needs to be blocked using a sound barrier not a vibration dampener. Dynamat Extreme is a dampener. It will help some but ultimately it will do very little to block road noise. They do have a product that is both dampener and barrier. That would be what you want. There are other options out there as well. Undercoating the wheel well on the outside will definitely help too. Sand and gravel hitting the wells creates a lot of noise and happens often.

What I've done on my Sierra and will do on my other cars is to lay down dampener pads/tiles, you only need to cover a percentage of the areas in question, followed by a decoupler layer which is typically closed cell foam, and finally top it off with mass loaded vinyl. It's important to cover the entire area with the foam and vinyl as any gaps are places noise will enter. My 01 Sierra is quieter than my sister's 05 Lexus RX330. I've covered the entire interior except the firewall and roof. Though I do have dampener tiles on the roof.