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SUVs Mazda CX-9 Mazda CX-9 Real World MPG
140 messages, Last post on Feb 13, 2009 at 12:14 PM
You are in the Mazda CX-9 Forum. Your Hosts are steve_ & tidester
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Replying to: bobw3 (Feb 10, 2009 2:02 pm) Yes, but basic statistics depends critically on using correct data. The departments of weights and measures in the various states regulate the accuracy of gas pumps to be about 6 cubic inches per 5 gallons. Since there are 231 cubic inches to the gallon, that works out to a relative error of just 0.5% per gallon pumped, not 20% If one is rounding off their numbers to the nearest 10th of a gallon then one is just being lazy. The price you actually paid for the gas and the posted price per gallon are sufficient to get an appropriately accurate measure of the volume of gasoline that you pumped. And, speaking of statistics, the "Law of Large Numbers" tells us that whatever errors you incur at each fill up will tend to the correct average over the course of time.  tidester, host SUVs and Smart Shopper
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Replying to: ceric (Feb 12, 2009 12:13 am) tidester, host SUVs and Smart Shopper |
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Replying to: ceric (Feb 12, 2009 12:13 am) It would be interesting (but very timeconsuming) to see measure the accuracy difference by only driving 0.2, 2, and 20 mile lengths. 40 27.3 26.4 25.4 24.7 35 26.7 26.4 24.8 24.7 30 26.4 24.8 24.5 24.7 25 25.6 24.4 23.7 23.5 20 24.3 22.8 22.8 21.4
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Replying to: tidester (Feb 12, 2009 1:16 am) Correct, but that's only the "fuel actually pumped" vs "what's shown on the gas pump gauge accuracy" What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time when they fill up their tank and then calculate their MPG. Even if the pump is 100% accurate, the point at which it clicks and shuts off varies a lot, depending on the speed of the fuel flowing into the tank, position of the nozzle, etc. Your original question was why it made a difference if you calculated MPG on a 2 gallong vs 20 gallon fillup and that's what I was trying to explain. It's the inaccuracy of the fillup (not the pump) that creates the error, and that error (which relates to the Law Of Large Numbers) will create a larger MPG inaccuracy the less you pump. Another way to think about it is to imagine if you drove one mile, filled up the tank and calculated MPG. The drive 100 miles, fill up the tank and calculate MPG. Which do you think will provide a more accurate result? Yes, it's an extreme example, but it shows that when trying to increase the accuracy of an MPG calculation it's better to drive more miles. |
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Replying to: bobw3 (Feb 12, 2009 11:57 am) The four numbers at the first row represents the MPG collected from various speed (35, 40, 45, 50 mph). Each data point is an average of four trials (back and forth twice). The ScanGaugeII computes MPG every 1 second or so. I reset the trip computer at the start and observe the average MPG at the 2-mile end. There are other factors that cause variations more than the ScanGaugeII such as the wind resistance. (on the day of our test, it was pretty windy) I also employed the cruise control to avoid human foot pressure variation on gas pedal. |
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Replying to: bobw3 (Feb 12, 2009 12:10 pm) Actually, that was Steve's question, not mine. What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time ... The information for doing it correctly is available based on what you pay and the price per gallon for your current fill up AND your previous one. Nevertheless, you have a point because most of us are not going to take the extra bookkeeping steps. tidester, host SUVs and Smart Shopper |
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Replying to: bobw3 (Feb 12, 2009 12:10 pm) Well, ok, but I live in a perfect world and Tides will vouch that my mpg spreadsheets are accurate out to 9 decimal points. 
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Replying to: steve_ (Feb 12, 2009 5:56 pm) Absolutely! But you never did tell me how you measured your fill ups to the nearest molecule or distance travelled to the nearest millimeter!  tidester, host SUVs and Smart Shopper |
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Replying to: steve_ (Feb 12, 2009 5:56 pm) My basic point was that point and short distances measurements aren't as accurate as those measured over longer distances.
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Replying to: bobw3 (Feb 13, 2009 8:49 am) It is interesting how the theory works out. I have a spreadsheet with ~400 tanks over a decade on my minivan and I could delete a few of them and it wouldn't affect the lifetime mpg. At least not out to one decimal point. 
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