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The first number is the width of the tire in millimetres. The bigger this number the wider the tire. More width = more grip, however, above 215mm the greater the possibility of rubbing.

The second number is the aspect ratio. This the distance from the bead of the tire to the tread of the tire as a percentage of the width of the tire. For example, a 50 series tire would have a sidewall height that is 50 percent of the width of the tire. The lower the sidewall heigth the less the tire will deflect in corners, however, the stiffer the ride.

The third number is the diameter size wheel in inches that the tire will fit on. Bigger wheels will give you lower sidewall height (to maintain same overall diameter) for better cornering, however more wheel and less tire weighs more and is slower in a straight line.

A 195/60-15 tire has a 195mm width, a sidewall height of (60% of 195mm) 117mm, and a wheel diameter size of 15". Double the sidewall height (117mm x 2) 234 and divide by 25.4 to get 9.2" plus 15" wheel to get a total of 24.2" total overall diamter.

In order to not affect the speedometer, try to maintain the same overall diameter. Otherwise pick a size that offers the characteristics and look that you are desiring.