When talking about measuring motorcycle wheels, you are probably talking about the rims, rather than just the tires. The tire sizes are printed on the side of the tire, and they do not really need to be measured; in fact, doing so might give you the wrong measurement. Rim measurements can be inferred by the motorcycle tire reading, but when you're trying to customise a rim, which many motorcycle enthusiasts do, the tire reading no longer holds the same value, since the rim measurements might change. Therefore, you must know how to accurately measure the rim, absent the tire itself.
Measure the rim width using your tape measure. The measurement should only include the distance between the insides of the rim, i.e. where the bead of the tire rests. Rim width is commonly and incorrectly measured from the outsides of the rim, but the true measurement is only the inside distance.
Measure the diameter from the insides of the rim, where the bead rests, across the full width of the rim. Again, a common error is measuring from the edges of the rim, but the resulting measurement will be too large.
Sometimes it can be difficult to measure from the inside bead rest. If that is the case, you can measure from the edges, but subtract the height of both rim lips. The rim lip can be measured by inserting a tape measure against the bead rest and measuring to the outside edge of the rim. You will have to do this on both sides and subtract the values from the edge-to-edge diameter.
Measure the distance from the edge of the hub to the edge of the rim by laying the rim on a flat surface so that it's balanced on the centre hub. Use your tape measure to measure the distance from the work surface to outer edge of the rim. Flip the rim and measure the other side. A discrepancy between the two measurements means the rim is not completely centred. This might be the case with rims that use spokes or on rims that have been bent.