First off, sorry for the size of the of font.
My iPad is so old and messed up (it's slowing down the entire Internet).
I can't copy text from any source and not have it appear as below, I've tried three different sources ie copied from the original source to Gmail then copied and pasted in here. Moving on.
If you've ever been to a circuit race of any type and been able to see a fresh seat of just off the track tire's you'll know they look like hell. The best place to see fresh race tire's has been in F1 coverage when the top three cars roll into their 1/2/3 spots during presentation.
In some of the shots you can really see all the worn marbles and junk the hot rubber picks up, not only tire marbles but anything in their path.
The follow is something I came up with 10 yrs ago. The key is to find a kit tire that crumbles and makes a dust, some tire's only shred and turn into little rolled up elongated bits of rubber this is not suitable for this method. You need dust.
1. Start with 120grit paper and sand the hell out of a spare tire so as to acumulate tire "dust". You'll need at least a tea spoon to start.
2. Mix white glue and semi gloss black acrylic paint. Sorry I don't know the ratio but it's more glue then paint.
3. On a flat smooth surface spread a small amount of the "dust"
4. Next using a flat wide brush, brush on a thin coating of the paint/glue.
5. Now roll the tire in the "dust" covering it completely,
6. Let the glue dry for a bit then roll the tire on a smooth flat surface to embed the dust into the glue.
7. Repeat steps 4/5 until you achive the look you want. Once the rubber starts to look compacted your on your way to achieving the affect.
8. Put on a final coat of black acrylic semi gloss, this give some sheen to the rubber making it look "fresh" off the track.
Final step is optional...you can also sand up different colour plastic and scrape off some grit of the 150paper and sparingly sprinkle a very very tiny amount on the tire's. This added affect appears as track garbage picked up by the tire's.
This really looks the part on an open wheel car.