text: Previous answer was a little incorrect.  In his example 195/55, 195 is the width and 55 means 55% aspect ratio, ie the depth of the tyre from rin to tread is 55% of the width (thus 107.25 mm in this case).  R refers to speed rating of the tyre.  Depending on where it has been manufactured there may be reference to max inflation pressure or also temperature.  To work out the dia add the wheel dia (16" in this case) to twice the depth calculated as above, making allowances that wheel dia is normally in inches  and the other dims are in mm.  R 16 does not therefore mean radius 16" but dia 16" and the tyre is an R speed rating.