One of the oldest driving jokes is about getting points for running over pedestrians. This joke take it to another level by referring to a driver's test which you are required to take and pass to get a driver's license. Usually you need to get about 70% in order to pass. In this case the wife tells her friend that her husband scored 8 out of 10, which could mean that he was smart and did well on the test. OR that he managed to run over 8 out of the 10 people he tried to kill!
