For every hour a vehicle is idling — which is when the engine is running but the vehicle is not moving — it it uses an estimated 0.2-0.7 gallons (0.75-2.6 liters) of gas. A common belief is that restarting a car’s engine uses more fuel than idling, but if a car idles for more than 30 seconds, it might be more fuel efficient to turn off the engine. Figures can vary depending on the specific vehicle, fuel cost and usage, but turning off the engine and restarting it instead of idling would save an estimated $70-$650 US Dollars (USD) a year.
More about fuel consumption:
Don't paint too broadly about diesel "trucks" wasting more.
My Ford Excursion's 7.3 Liter PowerStroke Diesel engine uses 0.55 gph with the AC running. Here in Texas, we might need to idle, even if it's more than 30 seconds, because it's too darn hot to sit in the car (at a traffic light, or waiting to pick up someone let's say) without AC. -- Lee in Central Texas.