Yeh man it can be a bit confusing, but for 4x4 purposes most refer to outer diameter of tyre as this is what counts when it comes to height and ground clearance, rim dia dont matter. The people that lable tyres dont make it easy, for example your tyres are 275/70 R16 275: The width of tyre in mm, a metric measurement. 70: The profile height, a percentage of width measurement. R16 : The rim size or inner diameter of tyre in inches, imperial.
It,s a bit like why do brown cows give white milk when they eat green grass?