CREATE TABLE table_78912 ("Season" FLOAT, "Date" TEXT, "Winning Driver" TEXT, "Car #" FLOAT, "Sponsor" TEXT, "Make" TEXT, "Team" TEXT, "Avg Speed" TEXT, "Margin of Victory" TEXT)

What was the average speed of Tony Stewart's winning Chevrolet Impala?
SELECT "Avg Speed" FROM table_78912 WHERE "Winning Driver" = 'tony stewart' AND "Make" = 'chevrolet impala'
SELECT "Avg Speed" FROM "table_78912" WHERE "Make" = 'chevrolet impala' AND "Winning Driver" = 'tony stewart'