According to this site (
http://www.bokchoys.com/differential/GearRatios.htm) you have a 3.46 diff ratio. However, pinch of salt here, because I have heard of the 3.23 ratio diffs are in the 2.5i Z4. Hopefully someone with a 2.5i will be along shortly to tell you exactly which diff ratio you will have.
Its very common for people with the 3.0i (myself) to switch from 3.07 to 3.46 to effectively shorten each gear. If you were to do the same you can go for the next most common sizes up which would be 3.64, 3.73 or 3.91. The link above tells you what diffs are on which vehicles,
but bear in mind it us for US vehicles and might not be the most accurate in the UK. Please check the list, then double check before buying a second hand diff. Ebay tends to be a good place to search.
Furthermore, if you are planning to change internals such as adding an LSD you need to make sure the diff type is the same. I believe they are BMW type 188 (sometimes 188K, 188L).
For example, some time in the future I plan to switch to a 3.38, 3.46 or 3.64 diff ratio meaning I could potentially source from the following: E36 and E46 3-series, X83 X3s and E85/E86 Z4s.
