I was looking at the new Shell fuel when I came across their advanced driving tips - one was on tyre tread depth and is quite startling:
"When tyre tread depth falls below 3mm your braking performance deteriorates, according to research by MIRA Ltd.
Tyres with a 3mm tread have a 25% better performance than those at the legal minimum of 1.6mm. In terms of stopping distance this represents an extra 8 metres, or 25 feet, in wet conditions. In one test, when a tyre with a tread depth of 8mm was compared to one with only 1.6mm, the stopping distance increased by 13 metres, slightly more than 42 feet.
The research has led to the majority of motoring experts recommending that motorists change their tyres when tread has worn down to no less than 3mm."
Personally I don't let mine get past 2mm or so and usually change well before the 1mm wear limit for motorcycles.