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Old 12-19-2007, 01:15 PM   #1 (permalink)
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How to calculate MPG / fuel economy formula

As surprising as it may seem to the hardcore fuel economy nuts who frequent Ecomodder, there are people who have never given much thought to the process of "manually" (ie. without benefit of a ScanGauge or other onboard device) figuring out a vehicle's fuel economy.

I had someone ask me how to do it yesterday.

After the initial shock wore off I came up with these instructions. Feel free to comment/add suggestions.

1) Next time you get gas, *fill* the tank. You can't calculate mileage if you're in the habit of just adding $10 or $20 worth each time. So fill until the pump clicks off once or twice.

2) Reset your trip odometer - if you have one. If you don't have one, write down your current odometer reading.

3) Drive, drive, drive...

4) Next time you need fuel: again, completely fill the tank until the pump stops. Get a receipt.

5) Write on the receipt the distance from your trip odo (or regular odometer) you went since the previous fill up.

6) Divide the distance driven by the amount of fuel used. That's your fuel economy. If your odometer is in miles, and the receipt shows gallons, the resulting number is your MPG (miles per gallon).

E.G. let's say you went 360 miles and you completely refilled the tank by adding 8.7 gallons (US). 360 miles / 8.7 gallons = 41.4 MPG.

7) Keep doing this each time you get gas. Save your receipts/calculations in your glove box or wherever. After a few fill-ups you'll be able to see what's "normal" for your car (assuming you do the same type of driving for each tank), and you can start using this info as a baseline from which you can try to improve.

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Old 12-19-2007, 01:25 PM   #2 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
1) Next time you get gas, *fill* the tank. You can't calc mileage if you're in the habit of just adding $10 or $20 worth each time. So fill until the pump clicks off once or twice.
Quote:
Whichever you choose do it the same every time
4) Next time you need fuel: again, completely fill the tank until the pump stops. Get a receipt.
Quote:
1 or 2 clicks whatever you decided on
Might want to say something about a shortfill not being accurate and once you have the FE bug try to use the same pump for every fill.
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Last edited by Lazarus; 12-19-2007 at 01:28 PM.. Reason: short fill
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Old 12-19-2007, 04:49 PM   #3 (permalink)
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Good points. For "moderate" fuel economy types.

You will know you've gone hardcore when you proceed to step 8: validating your odometer's accuracy and adding a correction factor in all your MPG calculations because it drives you nuts that it was off by 1.5%
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Old 12-19-2007, 05:30 PM   #4 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
You will know you've gone hardcore when you proceed to [B]step 8:
And I thought I was OCD. I'm just comparing to myself, basically, so I have skipped this step.

I like to do l/100km, so I just divide the other way, the number of litres by the number of kilometres (on my phone calculator), then shift the decimal point until it makes sense (two places).

The only problem with hypermiling is that it takes longer to get accurate feedback this way!
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Old 12-27-2007, 02:27 AM   #5 (permalink)
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I've been doing this for a little while now (though the car was off the road for a couple of months, so there's a hole in my figures) and it's quite interesting to see the trends.

I've been using a OO spreadsheet to track distance, fuel usage, cost, time between fillings, km/L etc, as well as the differences between values. Probably too many things, really, but it is interesting.


Here's the spreadsheet. Only seven entries so far, so the averages aren't the most accurate. And the numbers are all metric, and australian dollars. But it might be a handy template for someone else.

The EV bits on the right end are based on figures I've gotten around the intarwebs for comparison - one of these years I'll convert a car to electricity and test it out myself.

The file is .ods for Open Office, I can provide an .xls version if asked.
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File Type: zip Fuel Use Spreadsheet.zip (15.6 KB, 152 views)
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Old 11-13-2008, 10:46 AM   #6 (permalink)
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Ever since i signed up here I have been writing the mileage on my receipt and stuffing it into the glove box. One day I will actually pull them all out and enter them into the fuel log... i wanted to ignore it for a while so i could get a good back-trend. Maybe its that time.
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Old 11-13-2008, 11:38 AM   #7 (permalink)
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i love using excel to do all the work for me.

heres what i do:

lets start at line 5

A5 is where i enter the date

B5 is my calculated gas mileage
-enter " =D5/F5 " without the quotes

C5 is where i enter my trip OD miles for that tank

D5 is my "miles adjusted for tire size" formula (takes info from C5, its also the cell used for B5)

E5 calculates my cost per gallon (to keep from having to enter it myself every time)
-enter " =G5/F5 "

F5 is where i enter my gallons pumped

G5 is where i enter total fill-up cost

H5 calculates "cost per mile" for me.
-enter " =G5/D5 "

I5 is for notes


if you don't have mileage correction, just skip that one, and move your formulas over accordingly.

once you enter the formulas a few times (remember to change the number to whatever line you are on, and letters stay the same), excel recognizes the pattern and fills in for you.

i also did different width borders, put the mpg in bold text, and some other little things. i need to print it out and scan it as a .jpg, so i can post it up as a picture.


once its all set up, and remembers the pattern, keeping up with it is a piece of cake. i just fill-up, enter miles on the receipt, enter info into my computer, and throw the receipt into the recycle bin at work.



btw, the "cost per mile" has been really informative, almost more than mpg. when gas prices hit their summer/fall peak, i was getting my best mpg, and it was costing me .20 a mile to drive. now that the weather has gotten colder, my mpg have fallen. but so have gas prices. now i'm paying .10 a mile
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Last edited by Tony Raine; 11-13-2008 at 11:47 AM..
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Old 11-13-2008, 05:46 PM   #8 (permalink)
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Prior to getting my scanguage (just a couple weeks ago) I had been filling the tank past when the pump clicks (as much as 1 to 1.5 gallons). Now I am using the same pump, same station and only fill until it clicks. I suspect this will more accurately represent my mpg as filling past the click could be off by as much as half gallon every time I fill depending on how much time I spend trying to get it to max fill.

The only reason I filled past the click was to get another 2-3 days of driving in before having to fill up. With only an 8 mile roundtrip commute for work the fill past click saves a stop once or twice a month. But, to keep my sg2 accurate I'm just going to the click now.
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Old 12-03-2008, 03:16 PM   #9 (permalink)
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Since I got my current car 1 year ago, I also log all my fuel receipts in a spreadsheet (found elsewhere). As with the above MPG formula, consistency is key - always fillup to the same point.

So far I always forgot to write down the Current tank average on my ScanGauge before I reset it, so I can't compare its precision with the spreadsheet. Oh well, next tank, or the one after.
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Old 12-03-2008, 03:19 PM   #10 (permalink)
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Quote:
Originally Posted by Jeece View Post
So far I always forgot to write down the Current tank average on my ScanGauge before I reset it, so I can't compare its precision with the spreadsheet. Oh well, next tank, or the one after.
I do the same thing I'm always too excited to see what the error is, that I forget to write it down

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