I'd like to add sound deadening material to my car. How adversely will it affect MPG?

[redpoint5]

I wish aftermarket and even OEM headunits had active noise cancellation. The circuitry to do this is super easy and inexpensive, and all they would have to do is install a microphone somewhere in the cabin. It's the type of project you would find in a high school electronics class. Cars should have incorporated this 20 years ago.

[GearHead]

I think I'm just going to keep sound deadening mat in. It just doesn't seem worth it to stress over the percent gains I would possibly get back.

You all are right. The way someone drives and the routes someone takes has a bigger impact on the MPG. I should focus on that.

Thanks for all your help.