Simply "plugging tires" is no longer recomended by the manufactures.
The plug could leak internally and migrate thru the layers of the tire.
They actually recommend a patch and plug combo. Keep the air in and dirt out.
From the RMA,
Knowing the difference between a proper tire repair and an improper tire repair could be critical to vehicle safety. An improper tire repair could pose a safety hazard to you and your family and could also affect a tire manufacturer’s warranty.
One key process in a proper tire repair is removing a tire from the wheel to inspect any damage that may occur to the inner liner of the tire.
RMA offers tire dealers and automotive repair outlets a detailed wall-chart for proper tire repairs. Among the criteria to perform a proper repair are:[http://www]
Repairs are limited to the*tread area onlyPuncture injury cannot be greater than*1/4 inch*(6mm) in diameterRepairs must be performed by*removing the tire from the rim/wheel*assembly to perform a complete inspection to assess all damage that may be presentRepairs*cannot overlapA rubber stem, or plug, must be applied to fill the puncture injury and a patch must be applied to*seal the inner liner. A common repair unit is a one-piece unit with a stem and patch portion.*A plug by itself is an unacceptable repair