When a vehicle is coasting in gear, combustion doesn't take place inside the internal combustion engine. The wheels drive the engine through the transmission.
The concept of coasting in gear has been quite confusing to many especially when the intention is to conserve fuel.
Quite a number of people do coasting the wrong way; instead of coasting in gear, they coast in neutral. This does not cause conservation of fuel which they actually intend to do.
This article is for enlightenment/education because one will need a broken exhaust to carry out what we are talking about and we do not expect anyone to intentionally break a vehicles exhaust pipe to prove this. In the future, an opportunity to prove this might surface when you have a broken exhaust and you are on a road where it is okay to coast before you get to a garage to fix it. Note that it is illegal in some places to drive with a loud broken exhaust.
Exhaust note observed when coasting is done with gear in neutral.
When exhaust is broken and very audible and you drive with foot on gas, the note is high (very loud) but when you let off the gas, the loudness reduces drastically but you still hear the note clearly. Under this condition, it is obvious that combustion is still taking place in the cylinders and that cannot happen without the use of fuel.
Exhaust note observed when coasting is done in gear.
Now, when coasting is done with the vehicle in gear (the transmission not disengaged), you will notice that the exhaust totally goes silent when the foot is let off the gas pedal. This tells us that there is no combustion taking place in the engine's cylinders.
What happens during coasting in gear.
Our drive test with broken exhaust has clearly shown that when we coast in gear, there is no engine power used but the engine is driven by the wheels through the transmission.
When the gear is put in neutral, the wheels can no longer drive the engine through the transmission and the engine runs at idle speed, using the fuel needed to keep it idling.