How long can a gasoline car idle before it uses the same amount of gas that is required to start it? [Ray M.]
With respect to gasoline consumption, it is better to turn off your engine if you will be waiting longer than about 10 seconds. Beyond 10 seconds you will use more gasoline by idling than by restarting it. When first starting your car it is also better to drive it slowly as it warms up rather than let it warm up by idling. The latter also causes more wear on your engine that just driving it to warm it up. This is due to the longer time the engine is run at less than the ideal operating temperature resulting in incomplete combustion.
This is also true for modern diesel engines. Two exceptions are if a turbo diesel has been pulling a high load, it does require a cool down of a couple of minutes before shutdown to continue lubrication and the second is at very low temperatures batteries are less efficient and the oil and diesel fuel is thicker so constant restarts when the engine is cold may produce restart problems.