# Interesting experiment with fuel efficiency



## TXPearl (Apr 16, 2010)

There's been a lot of discussion in this forum about fuel efficiency, especially city vs. highway mileage. As expected, many have noted that city mileage is much lower, partly because of time spent idling at stop lights, etc.

I was curious to see what the fuel burn rate of the diesel engine is at idle. On an empty stretch of road, I slipped the transmission into Neutral and watched the instantaneous fuel economy gauge. When the gauge read exactly 50 MPG, I noted the speed of the vehicle. Dividing the latter by the former value gives you the gallons burned per hour at engine idle. I tried this both with AC and without AC. Here are the results:

AC on: 20 MPH / 50 MPG = 0.4 Gallons/hr.

AC off: 13 MPH / 50 MPG - 0.26 Gallons/hr.

This suggests that, over the course of a tank of gas, if you spend 1 hour+ at idle (AC on), you'd burn an extra gallon or so of fuel, or about a 6% hit to fuel economy (maybe 2 MPGs). Hard to say how much time I actually spend at idle.

Also interesting to note is how much the AC impacts fuel usage. For my test (I'm in Texas and the temperature was 100F), I'm assuming the compressor was engaged 100% of the time. In the real world, it would cycle on/off as needed and actual usage would be somewhere between the values cited above.

Since reading the fuel economy needle is not an exact science, I also attempted to estimate instantaneous mileage using the trip computer. The results seemed to match up pretty well with my gauge readings.

One other caveat - don't know if having the transmission in Drive while not moving would affect the fuel usage vs. idling in Park or Neutral (as I tested).


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## AutoUnion (Apr 11, 2005)

AC definitely does have a impact, but I never thought it would be so much. I've noticed it dropping only like 1mpg when the AC is turning on. I'd much rather get 1 less MPG then sweat my balls off in 85+F weather


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## 3ismagic# (Mar 17, 2011)

I live in Phoenix so driving without the AC this time of year is not an option.


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## TeddyBGame (Nov 17, 2010)

Thanks for posting this summary.

I live in Boston and rarely use the AC in my 2001 A6. I know it takes a huge hit on performance/efficiency. There are some days in the summer when it hits 90-95 where I need it...but most of the time, I crack the rear windows (especially on highway) to get some air circulation going.


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## KarlB (Nov 21, 2004)

TeddyBGame said:


> Thanks for posting this summary.
> 
> I live in Boston and rarely use the AC in my 2001 A6. I know it takes a huge hit on performance/efficiency. There are some days in the summer when it hits 90-95 where I need it...but most of the time, I crack the rear windows (especially on highway) to get some air circulation going.


yeah but down here in TX we are at 85 aT 7AM, at least you yankees cool off at night


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## craig1214 (Feb 16, 2007)

I would think the major hit to city MPG is frequent stopping and starting, and dumping kinetic engery into the brakes, then consuming fuel to get back up to speed, and repeat.


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## autoJeff (Oct 1, 2009)

Economy increases noticeably if you can avoid using brakes. For suburban driving, outside rush hour, it should be possible to use brakes very little. I think of "city" driving as situation where brakes must be used a lot, suburban is "mixed", and "highway" is large stretches of constant speed using cruise control.

I wonder how much loss occurs from accelerating through the RPM range where you are constantly handing off between smaller and larger turbos. Start off in a gear predominantly on small turbo. Large one begins to lose inertia. As RPM increases larger turbo spools up again. Delay shift until after large turbo is spooled up. Then shift and repeat.

If this loss can be significant then shifting earlier, but with same throttle input, might help economy.

But I suppose if the gears are close enough then this isn't a problem in practice. Maybe the closer gearing of the new 8 speed transmission helps here.


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## TXPearl (Apr 16, 2010)

craig1214 said:


> I would think the major hit to city MPG is frequent stopping and starting, and dumping kinetic engery into the brakes, then consuming fuel to get back up to speed, and repeat.


Agree. And I think my results corroborate that.

For simplicity, use the EPA mileage figures: 23 city, 36 highway - a difference of 13 MPG. You might lose a few MPGs idling but the bulk of the loss has to be from braking/acceleration.

As Autojeff has suggested, city driving can be much more efficient if you can avoid excessive braking.

Perhaps more important from this is, if you get stranded in a snowbank, you could last days with heat from the idling car. (If you started with a full tank)


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## dunderhi (Dec 10, 2006)

Braking - bad
High RPMs - bad
Idling - bad
Low Cetane - bad
Heavy wheels/tires - bad
Junk in the trunk - bad
A/C - bad

Coasting - good
Upshifting - good
Hard turning (ie no braking) - good
Proper tire pressure - good

Smoking the punk with the fart pipe - both good and bad.


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## AZ335D (Aug 19, 2010)

3ismagic# said:


> I live in Phoenix so driving without the AC this time of year is not an option.


I live in Phoenix (Scottsdale) as well. My AC button has been on for 2 or 3 months at this point, and probably will be until October or November.

We are in the middle of a cold spell - it only reached 103 today - had to get out my sweater.


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## AutoUnion (Apr 11, 2005)

AZ335D said:


> I live in Phoenix (Scottsdale) as well. My AC button has been on for 2 or 3 months at this point, and probably will be until October or November.
> 
> We are in the middle of a cold spell - it only reached 103 today - had to get out my sweater.


One upside to the warm weather is that diesels seem to love it. My old diesel always performed better in the summer than in the winter time. (also could be due to winter diesel with additives in it)


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## Snipe656 (Oct 22, 2009)

I have noted many times but for some reason my 335d guzzles fuel in comparisons to my other two vehicles when idling. Every time I have used the 335d for commuting and been stuck in my wifes parking lot for an extremely long time the mpg for that tank nose dives. This is not the case for the other two cars at all.


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## bimmerdiesel (Jul 9, 2010)

dunderhi said:


> Hard turning (ie no braking) - good


Recently a friend of mine had to change his axle on accord. He also drives like I do with very less braking on turns. Since then I am going easy on turns. In general what would be life of axle in BMWs?


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## 3ismagic# (Mar 17, 2011)

He may have ignored bad cv boots. Those are notoriously ****e on hondas.


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## dunderhi (Dec 10, 2006)

Snipe656 said:


> I have noted many times but for some reason my 335d guzzles fuel in comparisons to my other two vehicles when idling. Every time I have used the 335d for commuting and been stuck in my wifes parking lot for an extremely long time the mpg for that tank nose dives. This is not the case for the other two cars at all.


I'm addicted to hard turning, it's fun and much less likely to land a huge ticket vice the alternative method for having fun: speeding.

On my 5, I'm finding that I need to scrub my sidewalls more often. :rofl:


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## Pasa-d (May 7, 2011)

A few years ago I got pulled over by a cop. He came up to the car and said he swore I was up on two wheels when I made a right hand turn a block back. I said the speed limit's 35 and I was going 35. He had to concede that was true but said I should still slow down a little bit for turns :rofl: and let me off with a warning. This happened in my old non-DSC Audi S4 but I find the Bimmer to be just as flickable if you turn off the DSC.


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## floydarogers (Oct 11, 2010)

bimmerdiesel said:


> In general what would be life of axle in BMWs?


My front right axle-bearing in my E46 330xi went out at around 135K. The left and both rears was still good at 145K when I traded it in.:dunno:


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## Snipe656 (Oct 22, 2009)

Pasa-d said:


> A few years ago I got pulled over by a cop. He came up to the car and said he swore I was up on two wheels when I made a right hand turn a block back. I said the speed limit's 35 and I was going 35. He had to concede that was true but said I should still slow down a little bit for turns :rofl: and let me off with a warning. This happened in my old non-DSC Audi S4 but I find the Bimmer to be just as flickable if you turn off the DSC.


I know someone that got an exhibition of speed ticket for this sort of driving. Not sure how it stuck but supposedly it was the sound of screeching tires that did it for him.

My exhibition ticket(for loud and reving exhaust) was treated as one thing worse than a DWI by auto insurance. I'd rather have a few speeding tickets.

Sent from my iPad Nano


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## Flyingman (Sep 13, 2009)

TXPearl said:


> There's been a lot of discussion in this forum about fuel efficiency, especially city vs. highway mileage. As expected, many have noted that city mileage is much lower, partly because of time spent idling at stop lights, etc.
> 
> I was curious to see what the fuel burn rate of the diesel engine is at idle. On an empty stretch of road, I slipped the transmission into Neutral and watched the instantaneous fuel economy gauge. When the gauge read exactly 50 MPG, I noted the speed of the vehicle. Dividing the latter by the former value gives you the gallons burned per hour at engine idle. I tried this both with AC and without AC. Here are the results:
> 
> ...


TXPearl, I think I posted some data about this using my Garmin EcoRoute system. I get a readout of engine load in % and tried it normally with A/C in Drive at Idle, w/o A/C in Drive at idle, and with the car in neutral. It is very much measurable in engine load, which of course translates to mpg. My mpg generally drops slightly 1-2mpg during the summer months here in South Florida and I attribute this to the use of A/C.

So I'd say you are definitely in the ball park. Perhaps not scientific but definitely indicative.


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## TXPearl (Apr 16, 2010)

As a follow up to this, I have a 328i loaner for a few days. I repeated the same test with the loaner and it shows that the 328i uses roughly 20% more fuel at idle than the 335d. Somewhat surprising, as some have speculated that the diesel is less efficient in the city (vs. highway) because of excessive fuel usage while idling.


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## Philm35 (Aug 19, 2010)

> Economy increases noticeably if you can avoid using brakes.


It might be more accurate to say that "economy increases noticeably, if you can avoid accelerating after using the brakes". The brakes themselves don't do anything to cause fuel consumption.


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## Pasa-d (May 7, 2011)

TXPearl said:


> As a follow up to this, I have a 328i loaner for a few days. I repeated the same test with the loaner and it shows that the 328i uses roughly 20% more fuel at idle than the 335d. Somewhat surprising, as some have speculated that the diesel is less efficient in the city (vs. highway) because of excessive fuel usage while idling.


That's not surprising at all. Diesels are extremely efficient at idle because they can run very lean and have no throttling losses. City mileage in a non-storage power plant (non-hybrid) will always be lower than highway mileage simply because of the acceleration and deceleration.


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