Bonjour Daniel, J'aime bien ta voiture, elle est mieux que la mienne. I like your car, it's better than mine. In the phrase, obviously it is a comparaison, i.e. comparing ta voiture to la mienne. The word «mieux» is the adverb and «meilleure» is an adjective. So in comparing the two cars, the adverb is used for the comparaison. If the speaker were to be describing his own car in some detail, then the adjective would be used is the sense that I have on the difference. Bonne chance!