8x15 ET30
8 - width in inches (not actual width, measured bead seat to bead seat)
15 - diameter in inches (not actual diameter, measured bead seat to bead seat)
ET30 - offset from width middle to the mounting surface in millimeters

ET is abbreviation for EinpressTiefe (German, literally translated: Push-in depth)
So, assumming the same width, the smaller the ET number, the more a wheel goes outwards.


195/55 R 15
195 - width section in millimeters (195mm)
55 - height/width section ratio in percent (55%)
R - construction type (R=radial)
15 - diameter in inches (15")