Tire weathering can sometimes be a daunting task to some. How to get that "run in the dirt" used look. The method I use, while a little messing, produces a decent looking tire that I am proud to mount on my wheeled AFV's. It is an easy and quick way to get the dirt in the tread, and move onto to other things.
To start, I only use a couple basic weathering items, ok, well one, pigment. For pigment, I suggest use what ever works for you. For me, I think basically for the colour, I use Mig Dry Mud. I like the colour when dry, and if I need to vary the colour, I just add a little different pigment colour to get my desired shade. As for tools, an old brush, and a little container for mixing. For this I buy those little tart tins at the dollar store, and I continue to use the same one over and over again.
To get started, I have already painted the wheels, I usually use Vallejo Dark Rubber followed a wash using Vallejo Black Wash. It is also best if you have applied an aging effect and pin wash to the rims first.
Now onto the tire mudding. I take a generous amount of pigment and add it to the tart tin, and mix with water until it becomes a paste. Don't worry about adding to much water, as you can always add more pigment. AND don't worry about wasting it, just let it dry in the tin, and you can rehydrate it later for another use.
Now with the brush, make sure you are using an older brush, apply the "mud paste" to the entire wheel. Ensure you push the paste into the treads and everywhere else you want it to go.
Now set these aside to dry, I usually let it dry over night, but you can wait just a few hours if you are impatient. Once dry, this is the messy part, I rub the tire with my thumb to remove the excess mud, leaving as much or as little as I want, depending on the effect I am trying to achieve.
Once you are satisfied with the result, it is best to apply a flat clear coat to the tires to retain the effect. If you want to add some grease stains to the rims, I suggest doing it before the clear coat, as you can achieve the effect of the dry mud absorbing some of the grease. For grease, I use Mig 502 Abteilung Oils, Engine Grease. I just thin it down with a little white spirits, and apply like a pin wash.
Once you have the clear coat done, the tires are dirty and ready for your wheeled AFV.
Good luck and happy modelling.