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I am curious how much gas my 2016 28i 4 cylinder X3 burns when idling in gallons/hour.

There are certainly plenty of hits when googling this question but the hits vary widely. I am guessing about .25 gallons per hour.

I became curious as the result of a recent news article I just read about a guy and his dog that survived for five days in a snow bound SUV by eating packets of taco sauce. It was reported he periodically ran the engine to produce heat. This made me think how long the 17.7 gallon tank will last with the engine idling.
 

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It's actually easy to measure. All you need is a stopwatch. Reset your trip odometer and either on-board computer (OBC) MPG or trip computer (TC) MPG at the same time. The amount of fuel used from reset to any point in the future is miles/MPG.

Assume that the MPG display is rounded, e.g. 24.56 MPG would be displayed as 24.6 and 24.53 MPG would be displayed as 24.5. That means that at the instant the MPG display goes from 24.6 to 24.5, the calculated MPG is actually 24.55.

Use the stopwatch to measure the time it takes (t) the MPG display to change to a starting value (MPG1) to an ending value (MPG2). Because of rounding, and the fact that you MPG since reset always goes down when you're idling, the actual MPG at the start will be MPG1+0.05, and the actual MPG at the end will be MPG2+0.05. Example: Start timing when the MPG meter changes from 24.6 to 24.5. At that instant, the actual MPG will be 24.55.

The fuel used from the trip odometer and MPG reset to the start of timing will be (trip odometer miles)/(MPG1+0.05). Similarly, the fuel used from the trip odometer and MPG reset to the end of timing will be (trip odometer miles) / (MPG2+0.05). So, the fuel used during time (t) is the difference: (trip odometer miles) / [1/(MPG2+0.5) - 1/(MPG1+0.5)]. The fuel use rate (gallons/hour) at idle would therefore be: [(trip odometer miles/(t)] / [1/(MPG2+0.5) - 1/(MPG1+0.5)].

As with many things in my life, I put the logic above into an Excel spreadsheet. Here are data points for my F10 535i and my Chevy Cobalt (which has a more accurate MPG display than all of my BMW's ever did). The fuel use at idle will vary with the engine being warmed up, the air conditioner being on, and in my case if I have DPH activated (since it has a hydraulic pump that the engine must power). My 535i's was 0.27 gallon/hour with the AC off, and 0.37 gallons/hour with the AC on. The Cobalt's with the AC on was about the same as the BMW's: 0.38 and 0.39 gallons/hour. All these values were with the engine already warmed up, though. Fuel use would be significantly higher at cold start-up.

BF's file attachment feature isn't working now. When it's back, I'll post a snapshot of my spreadsheet.

Well before the fifth day, I'd be thinking about making some dog tacos.
 

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02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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https://www.energy.gov/sites/prod/files/2015/02/f19/fotw#861_web.xlsx

My VW TDI idling at a stop light would cool from NOT to Cold indication. It was documented to use 0.17 gallons (643.52 milliliter) per hour diesel with no accessory loads and about twice that with HVAC. TDI are cold blooded beasts. At 180K miles it still produced 50+ MPG.
 

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A classmate of mine had a VW Rabbit diesel back in the 1980's. He lived way out in the boonies and commuted almost 100 miles to school every day. He was constantly reporting his 50+ MPG fuel economy. One day, we were expecting another MPG report. Instead, he said "My timing belt broke."

I need a new beater, and I get a special deal on GM stuff. So, my beaters are GM's bought new. The logical choice would have been a Chevy Colorado with a four-cylinder turbo diesel. But, for some reason, they decided that it needed a timing belt. WTF? When I read about that, I thought about my classmate 40 years ago.

MPG is a stupid metric. It makes a V12 Ferrari and Doug's VW seem better than they really are. The Fuel Used vs. MPG graph goes almost vertical at low MPG's (Ferrari... bad) and almost horizontal at high MPG's (VW diesel... good but not really accomplishing much). It's also why VW's aero (Cd=0.15), carbon fiber and magnesium body, XL1 hybrid-turbo-diesel (240 MPH Highway) was totally useless.
 
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