The phrase "stock rubber" refers to the original or standard rubber material used in a product, such as tires or other rubber components. It's the basic type of rubber that comes with the product when you buy it. Full definition
As I pointed out before though, don't expect to run stock rubber through the cones or down the strip if you want to be quick. (speedsportlife.com)
The original 1969 Camaro ZL1 would run low - 13s on stock rubber when other pony cars of the same generation wouldn't get close to that even on a pair of slicks. (automobilemag.com)
This Brake Stiffening kit prevents the caliper flex inherent in the stock rubber design. (humblemechanic.com)