I have a set of 18in fast trekkers on my 84, they slide like butter, but I wouldnt go off the gravel road with them, they are just too low, and they require major gear changing going to 18in. I would recommend 20's for the rear for fast trekkers. For the front, I have always been a big fan of the kenda front max. Ever seen one on the front of a tecate? It looks sweet, and handles awesome.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes