What's "running stock" mean?
Great question. The term "stock" means that the component of the car (in this case the tires) has not been modified, and come from the normal "stock" or inventory used to build every car of that make. When used generally, the term "stock", in relation to cars means that every component part in the car is an unmodified part that came from the manufacturer. So here the speaker is saying that the only reason the car is able to drive the way it does, is because the "stock" tires have been changed for something else. "Stock" is quite technical and is usually used by professional or people who are really into cars. You probably would not hear this term used in every day conversation. Ex: I built this car myself, it's not just a stock car. Ex: I switched my stock exhaust system for a dual rear exist exhaust system.