In my experience Tweed isn't a nice thing to get wet. Why ? The odour. Tweed is dyed with lichen and produces an unpleasant, though bearable smell when wet.
I inherited a reversible Aquascutum mac / raincoat, one side is cotton gabardine while the other tweed. Can i wear the "tweed side" on rain? can it repel/ resist water eventually?
You don't want to get caught in a torrential rainstorm in tweed (trust me on this), but it's fine in a drizzle. If you get particularly damp and then come into an over-heated room, it'll smell like wet wool for a minute or two. It takes much less time to soak through than worsted wool or cotton.