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

[GearHead]

Hey all,

I'd like to add sound deadening material to my car. I'm going to be using Spectrum (paint like substance). I was told the dry weight is about 70 lbs for the whole inside of the car with the floor only being covered. If it's a significant decrease in MPG, I'll probably only do the trunk.

My question to you all is, how badly will this hurt MPG? I already have about 40 lbs of audio equipment. Most of that is just a sub, and an amp. Is 70 lbs really that much more?

I have a Honda Civic VX.

[RPM]

Depends on driving conditions. When you are driving at a constant speed on the freeway, weight is not very important. Now if you are constantly accelerating and decelerating (e.g. city driving), weight becomes more of a factor.

[GearHead]

I live in a city, so there's a lot of stop and go. It's not like NYC, but it's similar.

I might start with the trunk first to see how the car sounds before doing the front.

[NoD~]

adding weight in a lot of stop and go is definitely the last thing you want to do, but there is going to be a small difference. Compensating how you drive with the weight is easy to balance it out and make your number drop insignificantly.

BTW: If you haven't sound deadened before, note that chances are, the difference will be miniscule for actual road noise (if that's your goal). I don't remember where, but I read how you can actually use 1/4 the amount of total deadener for the same resonance deadening effect. If you are looking for road noise reduction, there are a lot of more effective ways (but you'd likely be adding more weight). If you are after rattle reduction, it'll help to a degree, but rattle hunting every hurtz is your best bet (long process).

(Used to be a huge stereo guy, ran a stereo shop for a while. Do a lot more home audio now)

[MobilOne]

A lot of noise gets into the car thru tiny openings such as around doors and windows, or is generated by mirrors, wipers, and antennae. One way to find these leaks is to get a 3 or 4 foot rubber tube to use as a listening device and listen all around the passenger cabin while someone else drives. There are also sheets of sound absorber that one sticks on the inside of the door between the window and the outside skin. I'd try the rubber hose thing first to see how much noise is leaking in and where, as it works and the price is right.

[GearHead]

I'm not really having issues with rattles. The hatch glass does rattle a little, but that's only when I'm on poorly paved roads. My main problem is the drone of the road coming up through the back. I don't really notice any serious noises from the front of the car.

I've seen examples and I do know that by adding sound deadening material to the trunk and wheel wells, it'll eliminate a lot of road noise. That's really all I want to do. I just can't stand the drone when I'm on a highway.

By the way, if I only do the trunk it'll be around 30 pounds.

[RPM]

If your car is like mine, and I assume it is since it is basically the same car, just one generation older, most of the noise will come from the tires.

Generally speaking, the wider and sportier a tire is, the noisier it will be.
A set of quiet LRR tires in the original size will do wonders for noise and fuel economy.

Also, I've noticed that removing the parcel shelf noticeably increases cabin noise. If you have it off, try reinstating it.

Sorry for the somewhat off-topic nature of this post, I'm just trying to think of things you can do without adding weight.

[GearHead]

If I drive on a freshly paved highway, the car is pretty much quiet. I can enjoy it. It's very rare that you find new pavement in my state .

What is parcel shelf?

The tires that are currently on the car are Falken tires. Haven't checked what size, but they look stock.

[RPM]

Yep, that's tire noise.
Also, I forgot: as tires get old, they get noisier.

By parcel shelf I mean the piece that sits between the rear seat and window, covering the trunk area. Probably a British term, sorry.

[GearHead]

Do you mean the cloth fabric that lays over the back of the trunk?

I have that up at the moment. It's incredibly thin, so I can't see that dampening any noise at all.

*EDIT* The shelf is missing. We call that a cargo cover. The mounting points are somewhat broken. I tried mounting an aftermarket cover, but it didn't fit. That could have been the cover itself.

The spare tire lid was missing, but I ordered an OEM one from eBay.