Rob- I went from a 3.45 to a 3.91 in my '85 U.S.-spec car w/ automatic and haven't been sorry. The 3.91 was the ratio that BMW was installing in all later ('87-'88) 6ers in the U.S. If you do a lot of long distance highway cruising you might be happier with the 3.45, but if increased performance is your #1 priority go with the 3.91.