Re: The skinny on skinny tires...
What they said, re stick to original size, BUT, what is even more important is that you don't change the profile, ferinstance, your tire is likely a 130/80 which means that the height is 80% of the width. Going from a 130/80 to a 140/80 probably won't make much difference, other than clearance issues, but if you were to go to a 140/70 (if there is such a thing) it would cause major problems, as wide profile tires require wider rims. The major tire manufacturers have websites with the aforemention fitment guides for tires with recommended, minimum and maximum rim sizes. Use those and at least your choice will be safe. Needless to say manufacturers recommendations only apply to their products, so if say Michelin says a particular size of tire they sell is OK for you rim doesn't necessarily mean the same size dunlop is OK as well.