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BMW 3-Series (E90 E92) Forum > BMW E90/E92/E93 3-series General Forums > General E90 Sedan / E91 Wagon / E92 Coupe / E93 Cabrio > Averaged 42 mpg--is this even possible?



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      10-16-2009, 08:03 AM   #23
autoJeff
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Quote:
Originally Posted by UnknownSpiritX5 View Post
Drove 65 miles
What's the topography of that route? Is it constant elevation, downhill, how steep are the hills?

65 miles is not far. If a there is a significant downhill grade during that route your final average will reflect the benefit from the downhill stretch. If there are no appreciable uphill segments then I could believe that reported average. For a fair test you could have immediately turned around and driven back the reverse route...that would negate the benefit from changes in elevation and to some extent tail wind.
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      10-16-2009, 10:12 AM   #24
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as much as I like to see my mileage meter sweep up into the 50mpg range, I realized you need to do the math at each fill up to calculate the true mileage. All the variables eventually average themselves out by the time you empty the tank.
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      10-16-2009, 10:43 AM   #25
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The average mpg on the computer is pretty accurate. Reset it for a typical tank of driving. I average 21.1 mpg.
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      10-16-2009, 10:52 AM   #26
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Quote:
Originally Posted by mike-y View Post
as much as I like to see my mileage meter sweep up into the 50mpg range, I realized you need to do the math at each fill up to calculate the true mileage. All the variables eventually average themselves out by the time you empty the tank.
Exactly. You have to do the math. I surprised so many people seem to blindly believe that they have ultra sophisticated fuel metering devices providing this info. Go fill up your car, drive 400+ miles and fill it up. If it takes less than 10 gallons to fill it up, then I'll believe it.

Relying solely on the display is like predicting the weather using the outside temp gauge.
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      10-16-2009, 11:21 AM   #27
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Quote:
Originally Posted by Takashi View Post
I get my numbers off an excel spreadsheet using numbers from gas receipts from fuel stations and odometer of the car. The math is very simple and straight forward. You can easily figure it out with a piece of paper, a pen, and an ordinary calculator.

Where else do you think I get my numbers from?
From the little asian kid you keep locked in your basement...
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      10-16-2009, 01:12 PM   #28
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Out of curiosity, do you have an estimate on how much you save per year with all those calculations?
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      10-16-2009, 01:20 PM   #29
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^^ Takashi - I do the same, albeit for informational kicks only, not trying to save $$. My car is a DD, so I generally always go to the same Shell gas station near where I work, even the same pump. Looking back on the data, the best tank I've had is 27.9 mpg actual (computer reported 28.0 for that tank), back in June 2007. Basically all highway for that tank, but driving through the Shenandoahs, so quite hilly.

I discussed this in another thread, but the computer is inaccurate. In 144 tanks of gas, it's never gotten the mileage exactly right. In fact, it has always reported too high. On average, it reports about 0.4 to 0.5 mpg too high, but it has been off by 1.0 mpg or more several times.

Also, the computer rounds calculations in l/100 km, then converts to mpg, so you see strange rounding issues when you consolidate the data. Try sorting your "computer mpg" column on your spreadsheet and see how the numbers increase by 0.2 or 0.3 consistently, and the computer never reports the in-between numbers. Also, the gaps in reporting intervals grow as you get to higher mpg numbers, since it's rounding at vol/dist and then converting to dist/vol. It is inaccurate over and above that rounding error, though.

Anyway, this is uninteresting to most, but I thought maybe you, as a fellow gas tracker, might be interested.
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      10-16-2009, 02:56 PM   #30
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hmmm, someone is getting a costco membership for (insert whichever religious holiday you celebrate in december here)..
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      10-16-2009, 10:27 PM   #31
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So I guess I DID not average 42mpg. I probably should have stated where I got this reading anyway---my GPS system.

As for where I was travelling, it was on the LIE (I-495) traveling east bound from Jamaica to exit 62--that is, if anyone is familiar with that area. As for the terrain, I'm not sure if the whole way is mostly downhill, or uphill or even just straight.

I guess I'll be keeping an eye on it next time I travel. It was nice to think that I had 42 though =(
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