Probably your hear from the UK's foremost expert the minute I finish this post who will violently disagree with all that I post. But....

My 2p is that your probably right 3mil is the lowest I'd be comfy with on a tyre in wet slippery conditions (as I do a lot of high speed motoring) but I thought that the tread depth wasn't the main reason why Mud+Snow tyres gripped better. I believe it's the rubber compound itself thats the main difference, being softer and more suited to working at low temperatures.

Fuel Saver tyres having more silica in the mix which aids longevity but reduces rolling resistance. Rolling resistance being one major factor in fuel economy. That said just keeping your tyres up tot the right pressure and checking it every week is also going to help greatly with economy anyway. If it's a choice between a miniscule fuel saving and good grip personally I'll take good grip every time.

I think if your going to fit them -great. Keep them on till min depth and don't worry about changing them until then. Just keeping an eye on the pressures will do more for your economy than anything else.

Also remove roof racks, heavy clutter in your boot etc and drive a bit slower and gentle with the throttle. Thats going to help you a lot with economy.

I thought I would save maybe buying on eBay but everywhere locally wants to charge £18.50 per corner to fit them. Rip off Britain.

That's shocking. My local fitters are all around a tenner, including disposal of the old tyres, valve, balancing included.

But you don't balance them on your own machine?

>>>>also some places won;t fit 2nd hand tyres at all - any ideas why?