]: 'Speed Class and UHS Speed Class symbols indicate *minimum writing performance to ensure smooth writing of streaming content such as video shooting. This is important mainly for camcorders, video recorders and other devices with video recording capabilities. *Speed Class designates minimum writing performance to record video. The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10.' [*Emphasis added]
As stated, the Class (writing speed) is important in things like camcorders that are WRITING large amounts of data to the cards. Garmin units are not normally writing to the card, just reading from it. In further expanation, of 'The class (speed) of the card only indicates its write speed', this is in regard to the relative write speed i.e. Class 4 is faster than Class 2, 6 is faster than 4, & 10 is faster than 6.
Sorry Loly, i think you have misunderstood me.