Where do you think it came from? If it is there, it is in/on the tips. It just settles on the valance from air turbulence coming over the car. You can have soot on the tips and on the car, but not, on the car and not on the tips.

 

I have it too and it doesn't settle on the tips after I wash my car the next day I have to take a cloth over the back bumper because I can see it on my white car. I think it comes from having a none chambered exhaust. My car is stock with no tune and with stock exhaust i didn't notice it, then I had flowmater a.t. And didn't notice it but with the slp lm's I see it and I know that the stock tune is a little rich so unless you have a dyno to I think it will be present

 

I got bama tune (93)
no cats ,no soot either

 

I notice some of those saying they don't have it have cars which have colors that don't easily show the dust. I would suggest they drive their car all day and then wipe the top of the rear bumper with a white diaper, their finger tip or even a kleenex and then update their post.
On Screaming Yellow you can see everything, but maybe not on darker colors.
I've had the dust since day one with totally stock intake/exhaust.

 

Same here. I have a white car so the soot is very easy to see. I wouldn't have noticed it if the vehicle were darker. It's been the case since the car was 100% stock.

My relative has a stock C6 Corvette and he's noticed the same thing. He seems to think it's brake dust.

 

If inside your exhaust tips is clean and shiny and there is no soot in them then what you have on your rear bumper is road soot/dirt. Think about it, if what was on your bumper was coming out the exhaust then it would have to also be on and in your exhaust. If you exhaust is clean inside and out then the soot on the bumper is coming off the road. It is a mixture of tire wear dust, diesel soot, and just plain old dirt.

 

Possible... but if it's just road dust shouldn't all Mustangs have it, regardless of mods, richness, etc?
That's why we need all different colored Mustang's owners to do a check

 

Mine does the same thing, the bottom of my tips get black, and their is a 'soot' on the rear-end.

 

Same problem here with my performance white stang. Almost every day there is a light dusting on the bumper and my ducktail after i've driven a bit.

 

I have the same problem with dust on the rear of the car w/ the ducktail. I think many are confusing the "SOOT" which is black and comes from the exhaust with brake dust and many other things kicked up on to the car when driving. I have the side exhaust and only get it on the tips as mentioned earlier. I get dust everywhere including the rear but it is dust and not black soot or even brake dust from what I can tell. This is my experience with my car and may be different from others but I thought I would mention this as everyone gets dust on the back of there stangs esp with this body style. I cannot speak for the normal exhaust setup that exits the rear of the car and yes my previous stangs (87, 93 LX 5.0L ) with the long SS exhaust tips got soot on the inside and the tips of the exhaust but I never noticed it on the rear bumper or trunk area of the car. JMO