Drive/Neutral & Fuel Consumption

[Benzowner]

There have been a number of discussions on here regarding whether you leave your car in drive or neutral when at traffic lights etc, but does anyone know how much more fuel you use when in drive compared with neutral or park. After all, in drive, there is a load on the engine but the revs are mainatined by the electronics on the car. Just for interest which may lead to my changing my style of driving

 

[BTB 500]

I will take the extra 50p a year (or whatever) on fuel in preference to wear and tear on the gear selector etc., anytime!

 

[996jimbo]

I would guess the difference is insignificant. If you were really concerned you could consider switching off the engine when you stop - though re-starting might use more than what you save?

 

[Benzowner]

I am not really concerned, just curious. When I sit in traffic in my S320, the fuel concumption increases by about 1mpg per minute or so. I have not yet tried with the car in neutral. I know its not scientific, but just wondered if somone out there had some real info

 

[BTB 500]

I am not really concerned, just curious. When I sit in traffic in my S320, the fuel concumption increases by about 1mpg per minute or so. I have not yet tried with the car in neutral. I know its not scientific, but just wondered if somone out there had some real info

It will depend on the engine's idle RPM - the higher that is the more energy dissipated by the torque converter / transmission oil cooler (as heat).

 

[Fandango]

How can MILES per gallon increase when you are not moving.
The throttle pumps in extra fuel, so whether you are in neutral or drive the difference will be negligible.

 

[Benzowner]

How can MILES per gallon increase when you are not moving.
The throttle pumps in extra fuel, so whether you are in neutral or drive the difference will be negligible.

The fuel consumption increases, not mpg

 

[DieselE]

I used to own a Vauxhall Omega (3litre automatic) and the fuel computer had an instantaneous setting. At a standstill, the variation in fuel usage between Drive and Neutral was 0.1 gal per hour.

I'm with BTB500 on this, I'll risk any increase in fuel consumption to avoid increasing wear and tear on the gearbox.

 

[benno]

Even less of a concern running a Diesel, but still an interesting point.

 

[MBManInKen]

How can MILES per gallon increase when you are not moving.

There are 2 issues here:
(1) It's not the amount of "miles" that is increasing, it is the "miles per gallon" value that is decreasing. The OP said that the fuel consumption increases, in others words you cover less miles per gallon.

(2) The mpg value can be seen slightly simplistically as the result of a simple calculation:

C = V / s

with C = consumption in mpg
V=distance covered in miles
s=quantity of fuel used in gallons

Since even at standstil you consume fuel when the engine is running, the amount of fuel increases, in other words while V in the formula remains the same, s increases slightly.

As s increases, the value of C obviously decreases for the same value of V, in other words: your fuel consumption for the trip has increased, shown by a decrease in the mpg result since you would cover less miles for a given quantity of fuel.

All perfectly logical

 

[BTB 500]

Since even at standstil you consume fuel when the engine is running

Of course even if the engine isn't running your fuel consumption for the trip can be affected - by operating anything electrical (because as soon as you start, via the alternator the engine will have to put back into the battery what you took out ... which takes additional effort)

 

[fuzzer]

Of course even if the engine isn't running your fuel consumption for the trip can be affected - by operating anything electrical (because as soon as you start, via the alternator the engine will have to put back into the battery what you took out ... which takes additional effort)

and if your using a diesel car , more than likely there will be an aux heater in the cold weather. And that uses fuel too

 

[MangoMan]

A Good Question....

There have been a number of discussions on here regarding whether you leave your car in drive or neutral when at traffic lights etc, but does anyone know how much more fuel you use when in drive compared with neutral or park. After all, in drive, there is a load on the engine but the revs are mainatined by the electronics on the car. Just for interest which may lead to my changing my style of driving

I am not going to answer your question directly, but many years ago a friend of mine, Sean O'Mant, his father used to be the only qualified RR mechanic in Zimbabwe, and funny enough I was round at his workshop with Sean cleaning our bikes and overheard a conversation about this between Seans dad and a customer. Now this customer was asking the same question because at the time fuel was a bit short at the time and this guy wanted to know if it would save fuel by putting it in N whilst at the Traffic Lights, etc.

Well, Seans father told him that after doing some calculations with a slide rule no less! He worked out that by leaving it in D whilst stationary was the same as cruising at approx 53kph on level ground.

Real funny you should ask the same question. Anyway, this is what he came up with in conclusion, I hope it somehow answers your question, although they were talking about a Roller that looked a bit like the one in the photo below, same colour strangely enough aswell.

 

[DieselE]

I am not going to answer your question directly, but many years ago a friend of mine, Sean O'Mant, his father used to be the only qualified RR mechanic in Zimbabwe, and funny enough I was round at his workshop with Sean cleaning our bikes and overheard a conversation about this between Seans dad and a customer. Now this customer was asking the same question because at the time fuel was a bit short at the time and this guy wanted to know if it would save fuel by putting it in N whilst at the Traffic Lights, etc.

Well, Seans father told him that after doing some calculations with a slide rule no less! He worked out that by leaving it in D whilst stationary was the same as cruising at approx 53kph on level ground.

Real funny you should ask the same question. Anyway, this is what he came up with in conclusion, I hope it somehow answers your question, although they were talking about a Roller that looked a bit like the one in the photo below, same colour strangely enough aswell.

That's an interesting statistic but there are a few caveats. Even the fuel used to run the engine in neutral will be sufficient to maintain a steady state speed over level ground.

In Drive, without touching the throttle my E320CDI will accelerate up to between 5 and 7mph. If I could rig the system so that the same throttle setting existed on overrun it would maintain a much faster balancing speed.

No doubt someone in Mercedes-Benz will have measured data for the fuel used at tickover both in neutral and drive for every MB model and that is the only true comparison. I doubt that the difference is that dramatic.

 

[MangoMan]

Oh, I nearly forgot, one other thing he mentioned was that, that according to his calculations also, obviously knowing the economy, (or lack of it), of the car, I remember him saying something that for every 15 minutes stuck not moving with the car in Drive he would use almost 5 litres of fuel!

I think that's when I walked away, not believing a word, but thinking about it now, it seems to make sense, believe it or not!