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I've confused myself calc'ing my last tank :(

  #1  
Old 06-30-2005, 10:04 AM
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Default I've confused myself calc'ing my last tank :(

Thought I had this all worked out yesterday, but it's been bugging me...when I did the additional exercise I state below today, I'm now thinking I may have shorted my mileage when I posted it.

To wit:

I drove 571 mi (odo) on my last tank, which had been filled up to the brim (Xcel had asked me to do this so that he could get an indication of tank size on the HAH). IIRC, I put 17.5x gals in on that fill, which netted me the 571 mi.

However, I'm a 'first click' guy, so I only put in 13.44 gal to fill two days ago, (plus there was 0.5gal added during the prior tank as a result of carrying around that much when I was draining the tank before that one as much as possible when I put the 17.5x gals in. Since I didn't want to waste that .5 gal, I added it to the tank as I drove on it = 13.9 gals to fill up. Right ?

Now, if I just do the standard "miles / gal to fill", that would give me ~ 40.7mpg - outstanding, and my best tank by far - but I know that's not the case. So - when I posted the tank, I used the averaging mpg readout from my trip odo, which read 34.2 - still (one of) my very best tank, and generally, the trip odo is < 1mpg pessimistic. So that's what I posted, since the db allows me to do that (if you look at my car 'UzLess', you'll see my reference to this on my last tank).

Here's the part where I really get confused...
If I multiply 13.94 x 34.2, I get 476.74mi - way too little
If I multiply 17.5 x 34.2, I get 598.5 - close, but it's off by 4.8% - too much variance, in my book.

I know I didn't use 17.5 gals, because I was 60mi DTE, per the Navi's TC, and had 1/8th - 1/16th of a tank per the gauge - the only reason I filled up is that gas prices were seemingly increasing by the hour, now that we're near the holiday (exaggerating, but slightly) Also, the fill-up stated 13.44, as you'll recall (plus my gas can addition)

Should I edit the tank, or leave it as-is (the 16.x gals as reported in the DB was calc'd by the DB, since I input miles driven and avg - is *that* the right answer ?)

Responses appreciated - thanks!
 
  #2  
Old 06-30-2005, 10:26 AM
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Default Re: I've confused myself calc'ing my last tank :(

you used 13.44 + .5 + (volume between '1st click' and the brim is). It may be worthwhile to repeat the brim fill exercise to see how much it holds after '1st click.'

More than .75gal fits in there. I've never gone higher than that (I always top off to the next whole dollar) Probably about 1.2 max?? That still gives you >37mpg.

Any chance you used a different octane? Large temp change? A cold gallon is 'more' fuel than a hot gallon by mass, which is what matters (if it were convenient, we'd be buying fuel by the pound/kilo, not by volume).

Also, different station, different accuracy. Even same station, if the pump was calibrated during the interval.
 
  #3  
Old 06-30-2005, 10:38 AM
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Default Re: I've confused myself calc'ing my last tank :(

Originally Posted by gonavy
A cold gallon is 'more' fuel than a hot gallon by mass...
Note To Self: Always buy gasoline in the cold morning.

troy
 
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Old 06-30-2005, 11:05 AM
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Default Re: I've confused myself calc'ing my last tank :(

unfortunately day-to-day variances won't make much difference...underground tanks and whatnot. But a large enough difference sustained over a period of time could have an effect. Not large for our purposes, but its there.

Buying bulk fuel for ships, ambient temp variances make up ~5% difference. While refueling, fuel temp is taken periodically and the avg is used, then compensated back to 20degC to calculate how much to pay. Similarly, more than a little fuel gets 'burped' out to the overflow tanks on occasion because of heat expansion, transiting from colder water to hot water.
 
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Old 06-30-2005, 11:05 AM
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Default Re: I've confused myself calc'ing my last tank :(

Sorry - I should have conveyed that info to begin with - I used the same exact pump both times.

Also, for reference - and you'll see this on my tank-by-tank - I generally don't get more than 13.5 - 14 gal max on a refill (also, I generally go at least to the low warning light before filling; outliers are noted in my record)

Something's telling me that I 'used' 16+ as calculated, but then, I should have taken more than my (usual) 13.x at fillup, given my remaining range.

I also don't note this gas being used any faster than normal (indicating I got a 'short' tank this time around).

Thanks for the input - appreciated


PS: ElectricTroy - yep - always fill up in the cooler part of the day, morning preferred. Gas is more dense when it's colder (you'll get more in the tank). Sadly, I filled up in the evening time (both times, I believe - maybe I can find both receipts)
 
  #6  
Old 06-30-2005, 11:25 AM
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Default Re: I've confused myself calc'ing my last tank :(

Hi G&B,


In the usual way of doing things, the 17.5 gallon fill-up would be applied to the miles driven on the tank *preceding* the big fill-up, leading to a considerable UNDER-estimation of mpg. Then, your 571 miles driven followed by a usual fill-up would lead to a considerable OVER-estimation of mpg. These errors occur because our assumption that we are filling up the tank to the same point each time has been breached.

There is always going to tank to tank inaccuracy, because you cannot fill up to the exact same point each time. Consistency like stopping at the first click each time helps, but is still subject to differences, such as fill rates at different stations.

The odometer and gallons pumped are the most accurate data we have to go by, and variances in the 'full' tank correct themselves automagically in tank after tank data.

In your position, I would continue to to enter the odometer and gallons pumped data, and not worry about it. If you want, a comment that you overfilled one tank would clarify the abberant mpg to anyone scrutinizing your data
 
  #7  
Old 06-30-2005, 11:57 AM
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Default Re: I've confused myself calc'ing my last tank :(

Originally Posted by ElectricTroy
Note To Self: Always buy gasoline in the cold morning.
Well fortunately (or unfortunately) since the fuel is stored underground, there's very little variation in fuel temp based on time of day.
 
  #8  
Old 06-30-2005, 12:23 PM
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Default Re: I've confused myself calc'ing my last tank :(

EricGo - thanks. I understand the logic of what you're saying. What exacerbates my situation is that Xcel, prince among men that he is , filled up my tank before giving the car back to me a couple of weeks ago (and since he asked me to bring him the car w/ an empty tank, he effectively gave me a free tank of gas, and refused to take payment for it). Thus, I don't believe I have a 'baseline' for averaging the last two tanks.

If I adjust this one to 'actual pumped', giving me the over-reporting, I don't see how the next tank will give me 'under-'

Given that, I think the best thing to do is leave the tank as-posted - my routes didn't change much (if at all) over my usual, and I used the same pump, eliminating that variable. A little more air in the tires, a little more interstate, and a little bit left over in the ECU's memory from Xcel pushing the car to it's limit, and I think that gets me a couple mi/gal over my 'usual' - but not 7+.

Thanks to everyone for posting. +1 to reputation on 2 individuals in this post.
(ElectricTroy, I'd love to help dig you out of that -15 hole, but don't see how I can rightfully do so on this post, at least. Truly, I'm starting to wonder if you've wronged someone in a past life)
 
  #9  
Old 06-30-2005, 01:02 PM
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Default Re: I've confused myself calc'ing my last tank :(

Mark,

For lifetime mpg accuracy, you only have to include all gallons of gas put into the tank (and it does not matter if some was taken out and then put back -- just count it once).
 
  #10  
Old 06-30-2005, 07:37 PM
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Default Re: I've confused myself calc'ing my last tank :(

Hi GreenAndBlue:

___If you have ever had the chance to look at some of the tanks of the Insight, you will see a few instances when I could vapor recovery fill on one tank and then the next tank I could not. When looking at the FE of each, the first would be extremely low (~ 70 mpg) and the next, extremely high (~ 135 mpg). What I did in those instances was to average the two. I knew exactly how many gallons I filled for the two tanks. I know exactly how many miles I had driven over the two tanks. Thus an average of the two is an exact average. It screwed the maximums a bit in the DB but that is OK as I preserved those with pics and detailed a description.

___Here is an example:

Notes: 06/29/2004: 1523.9 miles on 11.238 gallons = 135.60 mpg. Added this and last tanks distance / divided by total gallons: (1153.1 + 1523.9) miles/(13.895 + 11.238) gallons = 106.5 mpg due to the vapor recovery HW fill

___This should work well in your situation as well.

___Good Luck

___Wayne R. Gerdes
___[email protected]
 

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