Although the legal limit is 1.6mm of tread across the width of the tyre most organisations suggest that once you reach about 3mm it is time to think about changing.
There is a huge drop off in performance, grip, effeciency under braking as a tyre tread wears down for example it takes you twice as long to stop in the wet with 3mm of tread than when a tyre is new.
Don't take a chance with safety, we will happily inspect your tyres for FREE and give you our hioonest opinion.