My 13" retina macbook card reader SD has link speed @ 2,5GT/s. Most of us are used to seeing bus speeds specified in Gbps, or gigabits per second, but GT/s stands for gigatransfers per second. The difference has to do with the encoding of the data. Because PCIe is a serial bus with the clock embedded in the data, it needs to ensure that enough level transitions (1 to 0 and 0 to 1) occur for a receiver to recover the clock. To increase level transitions, PCIe uses “8b/10b” encoding, where every eight bits are encoded into a 10-bit symbol that is then decoded at the receiver. Thus, the bus needs to transfer 10 bits to send 8 bits of encoded data. A single PCIe 1.1 lane, the bidirectional bus can transfer 2.5 Gbps in each direction, or 5 Gbps in total. Because the bus needs to send 10 bits of encoded data for every 8 bits of unencoded data, the effective bit rate is 5 Gbps x (8/10), or 4 Gbps.