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Discussion Starter · #1 ·
Sure I have asked this before but someone has asked me again.

Does it use less fuel to take a car out of gear and let it roll down a hill with the car idleing in neutral or does it use less to keep it in 5th with your foot off the gas?

A carb engine may use more when in gear if the engine is drawing fuel in but I dont know for sure about injection engines.

I really want to know if a 75 can save fuel by leaving it in gear while rolling.

Any thoughts?
 

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Formerly Trophynovabasher
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I thought you would save gas out of gear, idling speed is around 1000-2000rpm, whereas 5th will be 3000-4000rpm, therefore less fuel if idling? maybe?
 

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Discussion Starter · #3 ·
TrophyNovaBasher said:
I thought you would save gas out of gear, idling speed is around 1000-2000rpm, whereas 5th will be 3000-4000rpm, therefore less fuel if idling? maybe?
Yeah but if the car is moving at, say, a constant 60mph with no pressure on the pedal isnt less petrol being supplied to the engine as when its out of gear it has to keep the engine at tick over but rolling down a hill the engine needs very little or no gas to keep it turning over cos the fact that the car is moving in gear keeps things turning over.
 

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Formerly Trophynovabasher
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yeah you probably have a point there, i am no rocket scientist, if i was, i would be driving a rocket powered zr to the moon, so would be more concerned with Bernoulli's theorem.
 

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The ECU will be mapped to put virtually no fuel in whilst in gear with no throttle applied, letting it idle and coasting will ultimately use more fuel in the long run.

Steve
 

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Discussion Starter · #6 ·
Steve B said:
The ECU will be mapped to put virtually no fuel in whilst in gear with no throttle applied, letting it idle and coasting will ultimately use more fuel in the long run.

Steve
Ah ha, I knew it!
 

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Apparently this also applies to diesels as I've seen it written elsewhere, either on the old forums here on on xpower-mg.com.
 
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Just about all modern fuel injection cars will cut the fuel on a trailing throttle when going down hill or on over-run (ie the throttle is closed, engine in gear, and you are slowing down so your foot is off the pedal).

There is a throttle position sensor, and airflow transmitter and an engine speed sensor. If the ECU senses that the throttle position is closed and that the revs are above idle, then it cuts the fuel flow. This is to aid fuel consumption and prevent uneeded fuel entering the cylinders. Once the RPM meets (or drops just below) idle RPM then the ECU will re-enable the fuel flow to keep the engine at idle.

So a car going down hill with the engine in gear and the throttle closed will use less petrol than the same car going down hill with the engine out of gear and running at idle (plus you wont have the added bonus of engine braking either).

If you want to prove it for yourself, find a quiet empty road one day, accelerate away and get into 2nd gear, then just take your foot off the accelerator and let the car slow down. As the car slows and the engine RPM reaches idle settings, then you will get a lurch as the ECU kicks back in with the fuel and the car will to a degree drive itself maintianing that RPM. Many of you, as I do, probably use this effect when crawling along in traffic. Try doing it with an old carburettor car and see what happens!
 
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