晕 in 头晕 is yūn first tone but in 晕车 yùn fourth tone. I am fine with that but is there any rule around this or just something that you just learn and go on with? The normal tone changes I do know but this one i have just learnt by heart and not until now actually started to think about. Is it about different meanings maybe? tóuyūn is dizzy while yùnchē is carsick.