I was considering upgrading my SD card from a class 4 to a class 10, for the speed improvement. So, I first tested my card to see what it's current speed was.
My card is an LD ( class4 8GB, bought off the street in China:
Write: 11.7 MB/s
Read: 18.0 MB/s
If my understanding of speed class correctly, this is class10 speed, not class4.
So I tested my Chinese "Kingston" - mostly likely a "ghost shift" chip, also class4 8GB:
Write 7.1 MB/s
Read 14.7 MB/s
This is class6 speed, not class4.
So, if you buy counterfeit chips in China (or online from China), you can get a class10 chip for very cheap, I paid $13 for my 8GB card. Just test the speed before you buy.
So check the speed of your card before you buy a new one.