The size and write speed of the card should be of little consequence in theory. The format of the card [FAT or FAT32 only] and the size of the map image files their naming & placement is of great consequence [gmapsupp.img & gmapsup1.img only; img size under 4 Gibibyte (decimal Gigabyte); in Garmin root folder only]. The unit reads from the card but doesn't write to it in normal operation so the read speed isn't a concern. But that theory can go out the window with old Garmin units. My 710, 760, 765, 260W [hwid 0827] 255, 255W, 265W, 1xxx various and later will all use a Sandisk 64GB reformatted to FAT32, but my 260W [hwid 0737] will not.