Thread: Info concerning SDcard size and Maximum Tiles

A bit more on the relative writing speed of cards in the unit or in a card reader. Early Garmins have a 'USB1' connection capability only, which explains why write speeds are faster using a card reader with USB2 or 3 capability, i.e. the speed bottleneck is caused by the unit's USB write capability. Later units with USB2 capability can be limited by low speed class cards which of course cannot be written to faster than their relative write speed will allow.

USB1.1 write speed is 12 megabit [Mb] per second i.e. 1.5 megabyte [MB] per second [maximum]
USB2 write speed is 480 Mb/s i.e. 60 MB/s [maximum]
USB3 write speed is 5 Gb/s i.e. 625 MB/s [maximum]

SD Class 2 write speed is 2 MB/s [minimum]
SD/SDHC Class 4 write speed is 4 MB/s [minimum]
SDHC Class 6 write speed is 6 MB/s [minimum]
SDHC Class 8 write speed is 8 MB/s [minimum]
SDHC Class 10 write speed is 10 MB/s [minimum]
SDXC [UHS] cards have much higher transfer rates [both read & write in the case of UHS-II, however UHS-I SDXC and Class 10 SDHC probably have same minimum write speed of 10MB/s].

Looking at a couple of examples:

Your nuvi 660 has USB1: You put a 10 MB/s Class 10 SD into it and find it writes no faster than when you had a 2 MB/s Class 2. It can't because it's limited by it's USB1 to 1.5MB/s maximum, therefore not even capable of matching the Class 2 card's 2MB/s minimum write speed. Better to use a USB2/3 card writer which can easily exceed the write speed of any of those card classes, then even Class 2 will write noticeably faster than in the unit.

Your nuvi 3597 has USB2: You put a Class 10 in it and find it writes much faster than with a Class 2 [and relatively faster than with 4, 6 or 8 too]. But when you re-format an SDXC UHS-II [maximum theoretical transfer rate of 312MB/s] to FAT32 and stick that in it can't be written to by the unit at any faster than the USB2 limit of 60MB/s. Maybe better to use a USB3 card writer which has 625MB/s maximum write speed so easily exceeds the 312MB/s theoretical transfer rate of a UHS-II card, but it has a practical minimum write speed of 30MB/s as a UHS-II card anyway, however the card is now acting like an SDHC and so it probably has the minimum write speed of 10MB/s of Class 10 or UHS-I anyway.

It's actually not easy to compare write speeds because the standards vary. USB is given as maximum write speed. SD/SDHC is given as minimum write speed. UHS classes are given as a 'maximum theoretical transfer rate', which I dare say is in fact a read speed and therefore actual write speed is considerably less both 'theoretical' and in real-life. For UHS-I it seems the same as Class 10.