Secure Digital (SD) card Speeds

Speeds
There are different speed grades available, measured the same as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s). High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 200x or higher.

SanDisk classifies their cards as:
* Ultra II—minimum read speed of 15 MB/s (100x)
* Extreme III—maximum speed of 30 MB/s (200x)
* Extreme IV—up to 45 MB/s (300x)

Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.

Capacity  Standard SD: 1 MB to 2 GB

SDXC (2009) – 64GB to 2TB
SDHC (2006) – 4GB -32 GB
SD (1999) – up to 2GB

SDXC cards are formatted in the exFAT file system, which allows for files of virtually unlimited size. (SDHC, which uses FAT32, can only record files up to 4 GB.)

Rating	Speed (MB/s)	SD Class
6x	0.9	n/a
10x	1.5	n/a
13x	2.0	2
26x	4.0	4
32x	4.8	4
40x	6.0	6
66x	10.0	6
100x	15.0	6
133x	20.0	6
150x	22.5	6
200x	30.0	6
266x	40.0	6
300x	45.0	6

Wikipedia