SDHC cards have 3 speed classifications; classes 2, 4 and 6 which correspond to write speeds of 2 4 and 6 Mb/sec. Why then are some SDHC cards rated as class 4 (or even 2) with write speeds greater than 6 Mb/sec. For example, Sandisc has class 4 cards with 15 Mb/sec rates called class 4 cards.