Andyboy999 wrote:Noooooooo! Don't fit a 3.91. It won't go much better but it'll rev like buggery and use a load more fuel/nerves.
One of the best mods I ever did to my Touring was to replace the 3.91 with a 3.64. Much nicer top end
,Just leave it well alone, they go well enough.

ive personally owned 6 x 325i's
4 of them with 3,64 diffs and 2 with 3.91's
plus ive driven a fair few others
the 3.91 cars r noticeably more lively.
mpg is a little worse, but the gains outweigh the losses.
top end is unchanged as a 325i will not rev out in top with either diff.
but u will get to that top end figure quicker with a 3.91
even with 15 inch rims - is smaller than the 14 inch rim with 195/65's.
theres a 0.4 % difference
hardly earth shattering is it
at the end of the day its up to stu.
its a cheap (just time really) mod and it will give gains.
most performance increaes will involve a car using more fuel anywyas remember.
plus its something bmw did themselves to the phase 2 sports and to the tourings so it cant be very wrong can it ?
itd be better if ppl didnt give incorrect or misleading advice.