Audi: '89 audi 80 from 4 to 5 gearbox

Hello everyone, my excuse for bad english, I don't know the exact names for
parts. I have a a80 55kw k6 with 4 shifts.I think this is also the reason
why it's consuming a lot of gas..because it is make a lot of (revolutions?)
So I wonder if it is possible to replace the gearbox for one with 5 gears.
And what parts have to be replaced as well if i do this.. and what will the
cost be approximatly? I can buy a a80 '89 5 shift gearbox for 50 euro's
Anyone experience with this?? Thanks. Greets Mark