Michael,
I hope you don't mind me showing you the method I used to get the wheels and tyres dusty on my Famo.


I used a set of resin wheels and tyres but the method is just the same for all, I even use it for braille scale kits.
The wheels and tyres were first of all sprayed with Tamiya Nato Black. Then the wheels were sprayed with Tamiya Dark Yellow so that they were all finished but still looked brand new!! I mixed some Mig Pigment Europe Dust with enamel thinners and with a largish paint brush, applied it over the wheels and tyres so that they were completely covered. Leave it to dry completely.
The next bit is tricky to describe but I will give it a go!
The idea is to leave the pigment paste only in the tyre treads, so it must be removed from the bit that would be in contact with the road surface and also the side walls. To start with get a cotton bud (do you call them Q Tips or something) and dip it in clean enamel thinners. Wipe / scrub the side walls of the tyres. Clean the bud and keep repeating until it looks ok.
Brush some thinners on to the wheels / tyres and wrap them in a small piece of rag or kitchen roll and rub it backwards and forwards between your thumb and fingers a few times. Do this for the sidewalls and the treaded area. This should leave the pigment paste just in the tyre treads and round the makers name on the side wall but not on the tyre itself.
If you are not happy with it, you can remove or add more paste until you get the look you are after.
Sorry about the long explanation but it's one of those jobs that is easier to do than to describe!
Hope that helps.
Can you tell me how you painted the rifle in the front passenger seat, and what colours / paints you used, please?
Andy