The boxes are interchangeable, however I am not 100% sure the ratios are the same. I remember a 146Ti I owned had different ratios to the 145 QV I replaced it with for some reason.
The individual gear ratios on a 145/146 2.0 TS box are identical, however the 145 has a final drive (differential) ratio of 1:3.562 while the 146 has a final drive ratio of 1:3.353. So the gearboxes fitted to a 145 2.0 TS and a 146 2.0 TS are actually different.
On the 1.4 TS the individual gear ratios are identical for the 145/146 and so are the final drive ratios. However, there were actually 2 different gearboxes fitted to the 1.4 TS engined cars - a C.510.5 and also a C.513.5. The C.513.5 was a revised version that was fitted to the later 1.4 TS engined cars. Both gearboxes have identical individual gear ratios but the C.510.5 has a final drive ratio of 1:3.866 whereas the later C.513.5 has a final drive ratio of 1:3.933.
I won't be posting on this forum any more. If you want to know why and/or you want answers to some 145/146 related questions then please feel free to pop over to and post on the alfa145.com forum where you will find me with the same member name.
I have now disabled all my private messaging options on here but you can PM me over on the alfa145.com forum if you need to.
Not sure what one I have haha my car is a 00 model facelift. Your a fountain of knowledge GE was hoping you would reply.
In that case you have the later C.513.5 gearbox in yours (final drive ratio of 1:3.933).
If you get another gearbox off a plastic top 1.4 TS (either a 145 or a 146) then it will have the C.513.5 gearbox, so the same individual gear and final drive ratios as your current one It's the alloy topped engines which have the earlier C.510.5 gearbox.
The other difference is also the bell housing, the early models had 2 inspection holes for the clutch whereas the later only had one, also the earlier boxes were less prone to the mainshaft bearing failures which became more apparent with the selespeed system.