Ground clearance??

Thanks for that ,guys. Good feedback. Can you tell me what the maximum size I can fit on my current setup is? And why does bigger diameter make such a difference other than gearing?

Ive just started going to club events and Ive got 50/50 road tyres on it.

Fellas Need tyres for my 90 Surf, but so many to choose from??? I want a tyre that kicks ass in all off-road conditions but is street legal. Dont really care about the on road performance tooooo much but gotta be safe. Standard suspension with 15" wheels. Any thoughts?? BFG?? Pirelli?? Simex ...