A (Semi)-Interesting Experiment in Fuel Economy

nobbyv · 08-02-2008, 08:57 AM

If anyone cares, I was interested in the accuracy of the fuel economy display in the dash vs actual calculated fuel economy so I did a little experiment. I calculate fuel economy by filling up at the same station until the auto shutoff of the pump kicks in, dividing the trip odometer mileage by the number of gallons it took to fill the tank, resetting the odometer, then repeating.

Over three fill-ups, I have found that the dash display is consistently .5-.7MPG "optimistic".

Tank 1: 24.6MPG indicated, 24.0MPG calculated
Tank 2: 24.2MPG indicated, 23.7MPG calculated
Tank 3: 24.0MPG indicated, 23.3MPG calculated

The only X factor I have no way to calculate is if the auto shutoff on the pumps kicks in at EXACTLY the same point every time, but I doubt we're talking about more than a few tenths of a gallon, tops. Anyway, thought that was interesting. So while the dash display does not appear very accurate, it is at least reasonably precise if you're looking to do some pre- and post- SSTT or JB3 fuel economy comparisons.

Gray135i · 08-03-2008, 04:42 PM

Originally, using the same techniques you describe, the computer was accurate to less than a tenth of a mpg from actual. After resetting my computer, the mpg is off 2.3 mpg every time.

TP 135i · 08-03-2008, 05:47 PM

I did a composite for four fill-ups using the same method as you (same pump with the auto-off)
See results here.........(post #34)
http://www.1addicts.com/forums/showt...t=11385&page=2

ariesdriver · 08-03-2008, 07:59 PM

You can adjust the correction factor of the milage calculator in the "secret" menu. Just DAFS.

08-02-2008, 08:57 AM	#1
nobbyv Second Lieutenant 14 Rep 230 Posts Drives: 2013 Audi S6 Join Date: Oct 2008 Location: Derry, NH iTrader: (1)	A (Semi)-Interesting Experiment in Fuel Economy If anyone cares, I was interested in the accuracy of the fuel economy display in the dash vs actual calculated fuel economy so I did a little experiment. I calculate fuel economy by filling up at the same station until the auto shutoff of the pump kicks in, dividing the trip odometer mileage by the number of gallons it took to fill the tank, resetting the odometer, then repeating. Over three fill-ups, I have found that the dash display is consistently .5-.7MPG "optimistic". Tank 1: 24.6MPG indicated, 24.0MPG calculated Tank 2: 24.2MPG indicated, 23.7MPG calculated Tank 3: 24.0MPG indicated, 23.3MPG calculated The only X factor I have no way to calculate is if the auto shutoff on the pumps kicks in at EXACTLY the same point every time, but I doubt we're talking about more than a few tenths of a gallon, tops. Anyway, thought that was interesting. So while the dash display does not appear very accurate, it is at least reasonably precise if you're looking to do some pre- and post- SSTT or JB3 fuel economy comparisons.
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08-03-2008, 04:42 PM	#2
Gray135i Private First Class 15 Rep 138 Posts Drives: 2008 135i Join Date: Jul 2008 Location: East of the Mississippi iTrader: (0)	Originally, using the same techniques you describe, the computer was accurate to less than a tenth of a mpg from actual. After resetting my computer, the mpg is off 2.3 mpg every time.
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08-03-2008, 05:47 PM	#3
TP 135i Major 29 Rep 1,340 Posts Drives: 2008 135i SGM MT Join Date: Nov 2007 Location: Finksburg, Maryland iTrader: (0) Garage List 1995 Acura Integra SE [0.00] 2008 BMW 135i [0.00] 2006 Porsche Cayman S [0.00] 1999 Toyota 4-Runner [0.00]	I did a composite for four fill-ups using the same method as you (same pump with the auto-off) See results here.........(post #34) http://www.1addicts.com/forums/showt...t=11385&page=2 __________________ 2008 135i SGM
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08-03-2008, 07:59 PM	#4
ariesdriver New Member 0 Rep 18 Posts Drives: Join Date: May 2008 iTrader: (0)	You can adjust the correction factor of the milage calculator in the "secret" menu. Just DAFS.
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