These are Moto-Guzzi's recommended tire sizes. Using tires sizes that are different than the manufacturer recommended size is not a good thing to do for many reasons. Using tires that are larger, smaller, wider or thinner can have big effects on the way your vehicle rides and handles. Tires can rub, suspensions can bottom out, and/or sluggish or overly quick steering can be caused by improperly sized tires. Computerized components can also often be adversely effected by using tire sizes other than what the factory recommends. When you use tire sizes larger (or smaller) than recommended, your wheel will spin more (or less) times to cover the same distance of ground. This can possibly knock your speedometer, ignition box, fuel injection, and other electronic components out of calibration.