5mm? They've only had about 2mm of wear by then! Most places recommend 2 or 3mm. It's up to you really, how skint are you, do you drive a lot in the wet and so on. I change sooner rather than later with about 2or 3 mm but then I do about 20k a year just commuting.
Officially, change them when the tread is down to the wear indicators, but you'll get much better grip in the wet and snow if the treads are deeper.
It may be worth fitting winter tyres and then swapping them back later.
Personally, I wouldn't be changing my tyres at 5mm!
John
My main gripe is that a relation took her car in for an mot.
they told ther one tyre was ok but would be a good idea to replace seeing as winter was coming on etc... and so forth.
I'm certain that tyre had 4mm on it.
the one they put on is as khumo? tyre for £48
forgot tom mention the old tyre was a buget one, which had a significant (about 5mm deep) crease/dent forming on the tyre wall. don't know if it was struck or a manufacturing defect or what make it was, so can't decide if they did a favour of sorts or still ripped her off.
I feel a bit crap cos i recommended them