Question: What circuit had an Ensign N180 as the winning car in round 5?
SQL: SELECT "Circuit" FROM "2-10663439-2" WHERE "Winning car" = 'Ensign N180' AND "Round" = 5.0;

Question: How many rounds total were there on may 31?
SQL: SELECT COUNT("Round") FROM "2-10663439-2" WHERE "Date" = 'may 31';

Question: What was the average round for the Oulton Park circuit?
SQL: SELECT AVG("Round") FROM "2-10663439-2" WHERE "Circuit" = 'Oulton Park';