Muddy Tires?

Hello,
I'm having trouble finding a good technique for making tires on amored cars and anything thats related to the military look muddy/dirty . I used a "sludge" technique from an older article of the fsm magazine on a T55 Dragon Wagon, but the results were not what I wanted. The sludge technique was a mixture of water, dishwashing soap, and a thin type of model paint. Plus I used the technique on rubber wheels from the kit. Please tell me a good technique if you have one.