A method of controlling operation of a tire filling mixing machine, comprising: (a) mixing a polyurethane isocyanate and a catalyst in a first mixer to form a virgin polyurethane; (b) grinding cured polyurethane core bits in a grinder; (c) mixing the virgin polyurethane and the ground core bits in a second mixer thereby forming a mixed flatproofing material; (d) sequentially injecting the mixed flatproofing material into a series of tires; and characterised by (e) at time intervals between the injection of the mixed flatproofing materials into each one of the series of tires: measuring a ratio of ground core bits to virgin polyurethane in the mixed flatproofing material by: (i) weighing the virgin polyurethane prior to the virgin polyurethane entering the second mixer, and (ii) weighing the tire being filled that contains a mixture of virgin polyurethane and ground core bits; and then (iii) comparing the measured weight of the tire filled to the measured weight of the virgin polyurethane entering the second mixer, thereby calculating the ratio of ground core bits to virgin polyurethane in the mixed flatproofing material; comparing the measured ratio of ground core bits to virgin polyurethane to a pre-stored preferred ratio of ground core bits to virgin polyurethane; and then increasing or decreasing relative percentages of virgin polyurethane and ground core bits entering the second mixer by an amount that is a fraction of a difference between the measured ratio and the pre-stored preferred ratio, (i) wherein the fraction is between 1/4% and 5%, and (ii) wherein the increasing or decreasing of the relative percentages of the virgin polyurethane and the ground core bits entering the second mixer is only performed when the measured ratio of ground core bits to virgin polyurethane is within a desired error range. (a) mixing a polyurethane isocyanate and a catalyst in a first mixer to form a virgin polyurethane; (b) grinding cured polyurethane core bits in a grinder; (c) mixing the virgin polyurethane and the ground core bits in a second mixer thereby forming a mixed flatproofing material; (d) sequentially injecting the mixed flatproofing material into a series of tires; and characterised by (e) at time intervals between the injection of the mixed flatproofing materials into each one of the series of tires: measuring a ratio of ground core bits to virgin polyurethane in the mixed flatproofing material by: (i) weighing the virgin polyurethane prior to the virgin polyurethane entering the second mixer, and (ii) weighing the tire being filled that contains a mixture of virgin polyurethane and ground core bits; and then (iii) comparing the measured weight of the tire filled to the measured weight of the virgin polyurethane entering the second mixer, thereby calculating the ratio of ground core bits to virgin polyurethane in the mixed flatproofing material; comparing the measured ratio of ground core bits to virgin polyurethane to a pre-stored preferred ratio of ground core bits to virgin polyurethane; and then increasing or decreasing relative percentages of virgin polyurethane and ground core bits entering the second mixer by an amount that is a fraction of a difference between the measured ratio and the pre-stored preferred ratio, (i) wherein the fraction is between 1/4% and 5%, and (ii) wherein the increasing or decreasing of the relative percentages of the virgin polyurethane and the ground core bits entering the second mixer is only performed when the measured ratio of ground core bits to virgin polyurethane is within a desired error range. measuring a ratio of ground core bits to virgin polyurethane in the mixed flatproofing material by: (i) weighing the virgin polyurethane prior to the virgin polyurethane entering the second mixer, and (ii) weighing the tire being filled that contains a mixture of virgin polyurethane and ground core bits; and then (iii) comparing the measured weight of the tire filled to the measured weight of the virgin polyurethane entering the second mixer, thereby calculating the ratio of ground core bits to virgin polyurethane in the mixed flatproofing material; (i) weighing the virgin polyurethane prior to the virgin polyurethane entering the second mixer, and (ii) weighing the tire being filled that contains a mixture of virgin polyurethane and ground core bits; and then (iii) comparing the measured weight of the tire filled to the measured weight of the virgin polyurethane entering the second mixer, thereby calculating the ratio of ground core bits to virgin polyurethane in the mixed flatproofing material; comparing the measured ratio of ground core bits to virgin polyurethane to a pre-stored preferred ratio of ground core bits to virgin polyurethane; and then increasing or decreasing relative percentages of virgin polyurethane and ground core bits entering the second mixer by an amount that is a fraction of a difference between the measured ratio and the pre-stored preferred ratio, (i) wherein the fraction is between 1/4% and 5%, and (ii) wherein the increasing or decreasing of the relative percentages of the virgin polyurethane and the ground core bits entering the second mixer is only performed when the measured ratio of ground core bits to virgin polyurethane is within a desired error range. (i) wherein the fraction is between 1/4% and 5%, and (ii) wherein the increasing or decreasing of the relative percentages of the virgin polyurethane and the ground core bits entering the second mixer is only performed when the measured ratio of ground core bits to virgin polyurethane is within a desired error range.