How to average MFD MPG's

[SureValla]

Ok so you may or may not know that taking a average of multiple tanks MPG's is not the same thing as summing all the miles you drove and dividing it by the sum of all the gallons you've burned.

This has been bothering me since I got my prius two months ago. Ive wanted a way to take a lifetime measurement of my car's mpg via the MFD and by calculating it the old fashioned way. I finally found out how to do it:

Just to get our bearings lets go over what the old fashioned hand calculation is and means:

Lifetime MPG = Sum Miles drivin/Sum Gallons Burned

Now there are two ways to average the MFD mpg readings for multiple tanks

Method 1: Take an average of all the MPG's of each tank

For example say the MFD readings for 3 tanks is the following

44.2
46.2
49.4

Averaging these numbers gives you 46.6

Now this number is NOT comparable to the old fashioned method. This is because it was calculated differently. This number tells you not your lifetime MPG but what you can expect from your next tank of gas for you're car. You might say that is what the old fashioned way is too but this is not so.

Unless each tank is drivin the same number of miles old fasioned way and Method 1 will give different results. This is because your MPG is effected by how much gas is left in the tank (aka less fuel = less weight = better mpg).

To correct for this imbalence I present Method 2

Method 2: Weighted Average

Lets stick with the previous 3 tanks mpg and add how many gallons were burned on each tank

9.93
9.95
8.843

Now add a column to your spreadsheet. This column = that tanks gallons burned/sum of gallons burned on all tanks

so the next column will look like this for our 3 tanks:

.3457
.3464
.3079

Now add one more column and make it = Tank MFD MPG * the Weighted number just calculated for that column

this looks like this:

15.28
16.00
15.21

Summing these numbers will give you a MFD Average MPG that is comparable to the old fashioned number!

This gives 46.49

(note if you know how to use the sum product this is a little easier)

Summary:

Averaging multiple tanks MFD MPG is not the same as the old fashioned way

Meaning of old fashioned way: Your cars lifetime MPG
Meaning of Method 1: What you can expect your next tanks MPG to be on the MFD
Meaning of Method 2: Your cars lifetime MPG

Why Old fashioned and Method 2 do not equal Method 1: Method 1 and OFW give more weight then they should to different tanks of gas based on the number of gallons burned.

This difference becomes more obvious when you have more variation in fillup size (aka lots of small and large fillups)


Hope that was understandable and I helped somebody get over the annoyance Ive had for the last two months.

[goldprius]

My now deceased father taught me to keep a little notebook in the glovebox. Write down your odometer reading & gallons each time you stop for gas. It seems that way before hybrids, people would buy big pickups or sedans. Fill 'em way up on one tank, then stop at the first click on the next fill-up & brag about
what good mileage they were getting...

Sorry if that sound way low tech. But looking at six months or a year is sound.

I have heard talk of the Prius having a bladder. I topped off today. I had almost exactly 500 miles.
The mileage thing-a-ma-jingy said 62.7. I calculate 500/8 = 62.5 so that is close enough for government work.

I am sure I have not helped you, since you sound more computer savvy than me!

[Prius 07]

I use a spreadsheet to keep track of the statistics of every fill-up. To calculate the life-time average MPG as displayed by the MFD insert a formula that calculates it based on a weighted average. As you correctly stated a weighted average is required since mileage driven varies from tank to tank. I use a weighted average based on the mileage given for any given tank and the total lifetime mileage driven.

Once you have the correct formula just copy it down for every new fill-up.

[geodosch]

For years I've kept a small notebook in the side door pocket of each of my cars, and at every fillup record the date, odo reading, total $ amount & gallons purchased. This all goes into a spreadsheet to keep track of MPG, miles/week and year, and several other factors. I calculate total lifetime MPG, but what I find most useful is a column that shows the average of the last four tanks. I did this to smooth out the error due to every fill-up not bringing the tank to the exact same 'full' level.

(I actually started writing a perl cgi version of the spreadsheet, though I haven't gotten around to finishing that yet.)

[morpheusx]

or you can do it the super easy way and log it online at http://www.greenhybrid.com and it does it for you.

[geodosch]

<div class='quotetop'>QUOTE(morpheusx @ Aug 23 2007, 11:06 PM) [snapback]501363[/snapback]</div>

or you can do it the super easy way and log it online at http://www.greenhybrid.com and it does it for you.
[/b]

Don't you still have to type in the information? <_<

[morpheusx]

<div class='quotetop'>QUOTE(GeoDosch @ Aug 23 2007, 11:14 PM) [snapback]501368[/snapback]</div>

Don't you still have to type in the information? <_<
[/b]

all you have to type is
Miles Driven and MPG from MFD display can't get any easier then that

[geodosch]

<div class='quotetop'>QUOTE(morpheusx @ Aug 24 2007, 08:26 AM) [snapback]501485[/snapback]</div>

all you have to type is Miles Driven and MPG from MFD display
[/b]

If you're entering MPG off the MFD, then you don't really even need miles driven.

The reason I calculate it based on miles and fueling amount is that I know it's correct (within the accuracy of the gas pump meter & odometer, which I can calibrate.) I don't know how accurate the MFD is. And I've used this method on all my cars for years, most of which had no onboard MPG calculation.

With my spreadsheet, I'm entering 4 values, though I'm also getting a lot more information from that, such as how many miles I'm averaging per day/week/year, how much I've been paying, gas price trends, cost per mile etc.

<div class='quotetop'>QUOTE(morpheusx @ Aug 24 2007, 08:26 AM) [snapback]501485[/snapback]</div>

[...] can't get any easier then that
[/b]

Sure it could. There could be a link via my bluetooth data connection to automatically upload my mileage data.

[Bohous]

I didn't do a spreadsheet for my first year or so but recently started one just to test the accuracy of the MFD. I am doing it the old fashioned way by simply calculating each tank by hand and comparing my numbers to the MFD and come up with +/- average over time. I try to make a point to keep it as consistent as possible by filling up at 2 pips which is usually around 8 gallons. In the short time I have been doing this I have found that the MFD is generous but it varies so much tank to tank I expect that it might even out eventually. Given the argument presented by SureValla I might have to rethink this as an accurate method to test. I never thought I would take advice from a Yankee fan. :P

[Danny Hamilton]

<div class='quotetop'>QUOTE(SureValla @ Aug 23 2007, 06:40 PM) [snapback]501263[/snapback]</div>

Averaging these numbers gives you 46.6

Now this number is NOT comparable to the old fashioned method. This is because it was calculated differently. This number tells you not your lifetime MPG but what you can expect from your next tank of gas for you're car. You might say that is what the old fashioned way is too but this is not so.

Unless each tank is drivin the same number of miles old fasioned way and Method 1 will give different results. This is because your MPG is effected by how much gas is left in the tank (aka less fuel = less weight = better mpg).
[/b]

This number really doesn't tell you anything about what you can expect from your next tank, and the reason it doesn't give you the lifetime mpg really doesn't have much to do with the weight of the fuel in the tank. MPG is a ratio that tells you the relationship between miles driven and gallons used, but it doesn't tell you anything about how many miles you drove or how many gallons you used, so simply averaging the ratios only works if they are all computed relative to the same amount.

To calculate lifetime MPG when you regularly reset the MFD you need to keep track of any of the following:
MFD MPG at reset & gallons used for each MPG recorded (MPG * gallons = miles)
MFD MPG at reset & miles driven for each MPG recorded (miles / MPG = gallons)
total number of miles driven (odometer) & total gallons used (total miles / total gallons = lifetime MPG)
<div class='quotetop'>QUOTE(SureValla @ Aug 23 2007, 06:40 PM) [snapback]501263[/snapback]</div>

To correct for this imbalence I present Method 2. . . .
[/b]

It really doesn't need to be this complicated.

If you are keeping track of both the MPG and the number of gallons, then the math gets very simple.

MPG * gallons = miles
So for instance 44.2 MPG * 9.93 gallons = 438.906 miles driven on that tank.

Compute miles for each tank, add up all the miles, add up all the gallons, divide the two totals, and you have your lifetime MPG.

44.2 * 9.93 = 438.906 miles
46.2 * 9.95 = 459.69 miles
49.4 * 8.843 = 436.8442 miles

Total miles driven = 1335.4402
Total gallons used = 28.723

1335.4402 mi / 28.723 gal = 46.49376 MPG.