Obviously I will look stupid for this, but I run my tires way too much down on a road trip I’m currently doing. I’m driving 1700km (~1000miles) from the north of Norway to the south. The tires should’ve probably have been changed before I left. Either way, I’m stuck about 500km (~300miles) from my destination. Fortunately I managed to find new tires with the correct dimensions close by. But, moral of the story, don’t run your tires down like this.
On the side of the tire you have an indicator for the Thread left. It's called TWI, thread wear indicator. Once it's on level with the surface that touches the road, change tires.
Tires are cheap but don't cheap out