Only a few...Whether they are made in the same plant is yet another story...It is common for the same plant to be contracted to make the same product for different vendors. The companies you want to buy from are ones that own their plant(s), therefore, theoretically, quality control is higher as there is not a middle-man involved. Michelin, for instance, owns and runs all plants that make their tires, from materials (unfinished) to R&D to retread to building, mostly in South Carolina (for North America). Some off-brands may come from the same facility that makes another type of tire BUT that does not mean the same components are used (can happen) and the tire pattern cannot be the same. It's sort of like the situation in which the same plant that made shirts for Old Navy made them for the Gap and Banana Republic - same plant but different quality and craftsmanship of materials.