Larger diameter tire like adding teeth to the rear sprocket??
You have it backwards... It's the other way around..
Consider the extremes: Install a rear tire the diameter of a roller skate wheel (assume 2-inches.. It will have to spin really fast just to cover a foot (realistically impossible since the rear sprocket would be touching the ground, lets ignore that).. That's Low gearing!!! Top of fifth should get you 6.11mph . Now imagine that you install a tire with a Diameter of 10 feet. Each single rotation of the tire equals 34.4 feet traveled per rotation. Ignoring that it'd never fit in the swingarm, your top speed in the top of fifth gear is 341.95mph.
Basically, Larger diameter rear tire equals gearing up, since the distance traveled per rotation is higher.
If you want to gear down, using the thin rear sidewall technique, get a 110/80-18 or a 100/80-18 rear tire. Translated it means that the sidewall is 80% as tall as the tire is wide No sidewall would be something like a 100/0-18 (The knobs would be right on the rim.). a 5.10-18 tire is very close to a 120/100-18 tire when measured.
Yeah , I am a little backwards sometimes
Will a 10 foot diameter fit in my swingarm