Sound deadening is coming up in another thread, so I want to throw this out there for discussion.
I read an article a few years ago about auto soundproofing. The author maintained that simply slathering every surface with Dynamat was heavy, expensive and far from ideal. Instead, he stated that a single patch of Dynamat (or the like) on a large area of unsupported sheet metal (like a door skin) was all that's needed. Beyond that, a layer of foam surrounding the cabin would yield optimum soundproofing for the cost.
I filed the info away in my brain until one day I removed the door panel from my BMW. That car was easily the quietest car I've ever driven. I could whisper in the front seat to passengers in the back seat at 75 MPH. What I found in the door was exactly what the article described. There was a single, small, thin patch (maybe 1/8") of asphalt deadener on the door skin and a layer of light 1/2" foam behind the panel.
I've tried, but cannot locate that original article. Instead, here's another that outlines the same general process without as much explanation.
I know our 3g's have almost no sound deadening, so they might really benefit from a similar approach. It would be the ideal balance between fuel economy, cost and quietude.