Thanks for your reply Nathan. I've checked (it is on axle stands at the moment) and both wheels rotate forward. On the dirt, both wheels drive, so I'm pretty sure it is a LSD, so it is a bit worrying that everything I read online also says it should start with an S. I also took the guess that the ratio is 4.44. Is there a way to check it properly though? I've rung the local BMW agent, they don't know how to check it. Perhaps BMW Germany?