“The bottom line is that the sizing of tires for certain motorcycles is determined by the factory motorcycle and tire engineers. They know their science well and know what works best on certain motorcycles. They have devised a numbering system which is used when replacing your tire.”

Oversized Motorcycle Tires

Two links that explain:

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I’m a bicyclist…Cannondale. My thin tires are good for hard surfaces but not for off-road or mountain biking. It would seem that there are parallels between the two in the tire department.

You didn’t give much (any) description. What kind of bike? What kind of application? Chopper, cruiser, sport, dirt, drag, trick, other? Track, canyons, commuter? Make/model/year? Why are you thinking of changing from manufacture recommended size?

The bottom line is how do you want to change your bike’s turn characteristics? That’s the single biggest effect when changing tire sizes. Most people would not notice a tire size change, but if you’re pushing the edge of performance it can make all the difference in the world. I’d say 75 times out of a 100 that people want to go from a 180 rear to a 190 (or vice versa). You can argue all sides, depending on your application/bike.

Most people don’t realize that as you lean a bike over, you are effectively decreasing the radius of the wheels as the contact patch line gets closer to the rim (for a majority of bikes). So tire profile is a consideration as well as tire size in regards to adjusting handling characteristics. The tire profile is slightly different from manufacture to manufacture so tire brand is a consideration too. And so is rim width – which also changes the tire profile. In this last regard it’s important to know when comparing handling characteristics between different bike models that may have different rim widths.

low profile and as wide as your swingarm will allow for a street bike.