3mm left after only 20,000 miles and we suspect they are not wearing out early? too funny. try 70,000 miles that I have on my Long Trails. if your winters are not performing as long as these all seasons then they are wearing out more quickly (or they are not being taken care of)

I expect these Long Trails to last for a couple more years minimum as now that they are 6 years old the rubber is hardening. no dual compound tires that I know of for autos, but motorcycles have had them for a long time.

if money is not the critical factor, drive them all year. enjoy!

PS as JCL points out, don't lump snow and winter tires into the same category, they are different types for different purposes. also, if you are wearing out the threads of your tires, you have already completely worn away the tread (these are not the same things) and are now running on the belts!!