To choose the right tyres for your vehicle you need to know the correct dimensions about them. You can check the manual of the vehicle's producer or your registration certificate. Also you can look the sidewall of tyres fit on your vehicle at the moment.

185 - this is the normal width of the tyre in mm

55 - profile - it's not a height in mm, but percent ratio of the width, it should be choosen precise

R - for Radial tyre, all passenger tyres at the present moment are radial

15 - size of the Rim diameter in inches

81 - Load index - how many weight can carry one tyre, it defines by table, in this case 81 means 462kg

H - Speed index - what is the maximum speed you can drive this tyre, here H means 210km/h