Although dynamat is probably the best product on the market for noise reduction, remember 2 things. 1, it is expensive, 2, its heavy, and we all know weight means less MPG. It might not be much, but its a factor. Good Luck.
And another tip - most of the road noise in my Insight was coming from the rear well area.
As an experiment, I borrowed a heavy down comforter from the bedroom and spread it out over the cargo area, and was careful to not block the vents. Next commute to work I noticed this considerbly reduced the road noise. So I only put sound deadening in the spare tire/cargo well area.
Location: Oregon City, Oregon basically sub of Portland
Hybrids: '06 Civic HCHII Mag Pearl
Dynamat is for sound systems, it helps keep sound within your car. It does little to reduce sound from outside your car. Save your money on those expensive mat products.
I belong to the Corvette Forum and most Vettes are using a simple product you can buy from Lowes, or Home Depot. Frost King. It has thin aluminum on both sides and does an outstanding job of sound proofing. I have it in both my Vette's. Just use spray contact cement and go for it.
Link to give you an idea of what I'm talking about. This person pre-cuts the parts for installation in a Vette. Lesson's installation time.
First, thanks to everyone who replied, and for your great advice! I had the car "dynamatted". While the car isn't as quiet as I wish it were, it was a significant improvement. No change in gas mileage noticed from dynamatting, yet. BUT, I previously changed the stock tires to a more comfy-riding tire. My mpg drop from the tire change was 12%, but the car rides much better, and with the dynamatting it's an overall improvement. I know now that its "as good as it gets".
Hybrids: 2006 HCH II w/ Navi, 2010 Insight II EX w/Navi
I second the Frost King material. Bought it at HD here in LZ, along with a few smaller secitons that were self adhesive strips. Used in on my 2010 Insight and will install on my '06 HCH II. Finished a car for less than $50 between the metal tape, spray adhesive and insulation