Well I'll give you an example:
On my rear axle I had a couple of tyres both with 3.0mmm on them. Now I reckon to change my tyres at 2.0mm minimum. But it so happens one had a slow puncture [screw in it] and to be honest we did not know about it until it only had 8.0 psi in it. And that's only because it looked flat, possibly if we had been legging it on a fast A road or motorway we would have noticed the difference in the handling. But as it happens not done any of that for a week or so, mind you it just proves how good Focus are at handling and behaving on the road.
Anyway, the flat has scrubbed down to 2.5mm, now as the new tyre of my choice costs £73 with 8.0mm of tread on it. And I will run them until they are at 2.0mm that means 73 divide by 12 = £6.08 per 0.5mm. Now last time I had a tyre "plug" repaired it cost my £16.50 and as this tyre only has another 0.5mm of life left in it. I thought it better just to bring my rear axle tyre renewals forward, and run on new tyres for the winter period.
Now I have opted for the tyres that I bench marked in tyre research earlier this year [url="http://www.fordownersclub.com/forums/index.php?showtopic=9551&st=0&p=59815&fromsearch=1&#entry59815"]Four bits of rubber between you and whatever[/url]
And I must say at first glance on the car they do look up to the job. Now whilst I'm confident they will preform as well as the tests show on this [url="http://eu.goodyear.com/home_en/optigrip/tuv_sud_automotive.jsp"]web link[/url] We will have to see what the "Wear Rate" is like, and if they effect petrol economy.