T25 Diesel 5 speed upgrade....Help & advise please

[jokeruk]

If you're planning a TDI - make sure your gearbox is strong enough to start with. I'm going to be looking at TDI next year but will be budgeting for a rebuild by Aiden just in case.

[footstuck]

The various 5speed boxes have different FD ratios
I’ve just fitted a 5speed with a 4.83 C&Pinion, which could do with being a bit longer in the gears.
If your going TDI the longer the better. Although a 1Z with the right tune will rev, they feel so much better if they have longer gears to push against.
Don’t see an issue in changing down a cog when climbing, but agree that over gearing and being blown back down a hill would be a pain.
Aidan's your man for a suitable box build, book well in advance, he's in demand