Law of Diminishing Demand

Law of Diminishing Marginal Utility suggests that as we consume more products total utility of consumption increases continuously but the marginal utility decreases with each addition consumption. For example, a couple works in a commercial bank situated at a distance of 20 kilometer from home.  couple purchases a car to go for work. Naturally, the car will travel 40 kilometer daily. It means car utilization is 40 kilometer. If couple purchases second car to go for work separately, then both car has utility of 40 kilometer each, i.e. total 80 kilometers daily.  Due to rise in gasoline price, parking charges and toll the couple shall try to pool the car rather than driving separately.  Hence it is possible that second card hardly run 40 kilometers daily.  For a month, the average utility of first car is about to 40 kilometer but the average monthly utility of second car is less than 40 kilometer, say it is 20 kilometer. Now total utility both car is 60 kilometers but marginal utility of second car is less than first car.  Now, imagine what shall be marginal utility of third car if couple purchases. It may be either little utilize or has zero utility.