Dirt "dirt" or grass "dirt"?
For high speed, I think a lot of people are actually going with foams on the jato since it's so hard on tires.
For grass/dirt, I run 1/8 buggy tires/wheels, but now just put stampede tires/wheels on the rear. Not sure how it handles off road yet, I have snow and they suck in snow.
To get any type of "hooking up" in dirt, I ran losi step-pins on 2.2 wheels on the rear with losi blade tires on the front with 2.2 wheels.