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Discussion Starter #1 (Edited)
I have the 2014 Mazda3 Sport GT with 18" wheels and Dunlop tires and was sick of the road noise on the highway. I also noticed the noise was quite loud with winter tires on 15" wheels. So, instead of changing to Michelins right away, I decided to put some sound deadener from RAAM Audio in the hatch (around the spare tire). That helped, but I thought I could do better. So, I bought Spectrum spray-on coating from Second Skin Audio and sprayed that on the rear wheel wells and the front fender liners as well as a bit on the wheel well near the firewall where the factory missed. This had a positive impact as well and I measure the noise with a Radio Shack dBA meter. The results are as follows:

Cruise - New Asphalt - 60mph: 65dbA
Cruise - Old Asphalt - 60mph: 70dbA

When I change out the Dunlops for Michelin AS/3 I will re-test the decibel reading and update again.

On a subjective level, the difference after adding the sound deadening makes the car feel more relaxing to drive and have conversations in. It makes the car feel more solid and luxurious and I'm quite pleased with the results!

EDIT 5/14: I got my bodyshop to spray the underbody of the car with the same Spectrum coating (3 coats) and went to do some more dB testing tonight on the same patch of road in similar conditions and I did not get any better results. However, the floor does not vibrate from the tires as much anymore. I think it's time to change the tires as they are causing all of the noise (at 20,000 mi) as I believe the noise is mostly airborne noise from the tires as opposed to chassis-related harmonics now. It cost me a couple hundred cash to get the underbody sprayed. If you are serious about making the car as quiet and comfortable as possible, this is still a good bang for the buck IMO. If you want to just cut down on some road noise for cheap, just spray the fender wells and fenderliners.
 

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Thanks for sharing the experiences. Just changed to Michelin A/S at 10,000 miles on my 14 S Touring and subjectively it made a great difference on daily driving. Now the noise is much more tolerable but I'm still glad to hear that it makes a difference just covering the hatch area with sound deadener. I'll get some peel n seal and try to do that this weekend.
 

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Discussion Starter #3
I edited my original post with an update! Next stage is new tires in a month.

EDIT 5/14: I got my bodyshop to spray the underbody of the car with the same Spectrum coating (3 coats) and went to do some more dB testing tonight on the same patch of road in similar conditions and I did not get any better results. However, the floor does not vibrate from the tires as much anymore. I think it's time to change the tires as they are causing all of the noise (at 20,000 mi) as I believe the noise is mostly airborne noise from the tires as opposed to chassis-related harmonics now. It cost me a couple hundred cash to get the underbody sprayed. If you are serious about making the car as quiet and comfortable as possible, this is still a good bang for the buck IMO. If you want to just cut down on some road noise for cheap, just spray the fender wells and fenderliners.
 
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