When you are driving under load, you are using gas.
When you have your foot off the pedal and the car is slowing down (in gear) you are not using fuel. The engine is turning over because the momentum of the car is turning the gears which is turning the engine.
When you are out of gear or clutch pedal down, (of course with engine running), the engine is using fuel, regardless of whatever. This is because it needs fuel to keep running and since it's not in gear and there's no momentum keeping the engine running.
A test you can do: put the car in gear and coast down with your foot off the gas. After the engine reaches a minimum RPM, you can feel the ECU give it some fuel to keep the engine running. It does this because if RPM got too low, the engine would cut out and stall.
To recap, you are always using fuel except when in gear and coasting down with your foot off the gas.
If you go to just about any truck stop you will see most of the trucks idling. They get away with this because the amount of oil that is burning is minimal. Much less then in a gas engine. In a gas engine there is a throttle valve. The engine is a giant air pump so the throttle valve creates a restriction that the engine must over come. So in a gas engine it needs to burn gas to idle. That's the reason for the idle circuit in a carburetor. A diesel engine has no throttle valve so there is no restriction to the flow of air. So the amount of energy needed to idle the engine is greatly reduced. And very little fuel is burned to keep the engine idling with a diesel.
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