Sounds like Olds has you on the right track. My first thought that prompted me to ask you what ESC/Motor?battery you're running is because it sounds like you're experiencing some cogging issues. If thats what the problem is then a proper 1/8 scale ESC is needed.
Genrally on RCs the gearing is done through the pinion and spur. Some kits you might find some pinion gear for thats a different tooth count maybe even a ring ring but its not terribly common. I would think a 2000kV motor with a 3.06 gear ratio would run just fine even with the big MT tires...