Mile calc question

Try:


Total MPG = ( Leg1 Miles + Leg2 Miles [... + LegN MGP * LegN Miles] ) /

( (Leg1 Miles / Leg1 MPG) + (Leg2 Miles / Leg2 MPG) [... + (LegN Miles / LegN MPG) ] )

 

Hey tripp, you might want to loosen up that helmet a bit.

Tom

 

My formula is a bit different.

[segment 1 miles*MPG] + [segment 2 miles*MPG] / total miles = average tank MPG.

[53.2 * 377.5] + [55.8 * 93.4] / 470.9 = 53.71

 

<div class='quotetop'>QUOTE(IsrAmeriPrius @ Apr 24 2007, 06:29 AM) [snapback]428666[/snapback]</div>

your formula is not correct. Please try the below e.g. and you will see the big difference:

99 MPG/1 mi
50 MPG/1 mi

Using your formula:

[1 * 99] + [1 * 50] / 2 = 74.5

Using the correct formula:

2 / ([1 / 99 ] + [1 / 50 ]) = 66.4

 

It's not that there's some "magic" formula that you need to use to calculate this. Remember that MPG stands for "miles per gallon," or, in other words, miles/gallons (miles divided by gallons). That means, if you know the MPG and the miles, you just need to divide the miles by the MPG and you'll get the gallons used:

a miles / x (miles/gallon) = z gallons

Then, you just add up your total gallons, add up your total miles (which was known information) and divide the miles by the gallons. Bingo! Total MPG!

(a miles + b miles) / ((a miles/ x mpg) + (b miles / y mpg)) = Total MPG

 

<div class='quotetop'>QUOTE(vincent1449p @ Apr 24 2007, 04:33 AM) [snapback]428916[/snapback]</div>

Except that I got the exact same result that Dr. Fusco did when he first calculated the total number of gallons used and then proceeded to divide the total miles driven on that tank by the number of gallons.

 

<div class='quotetop'>QUOTE(IsrAmeriPrius @ Apr 25 2007, 09:09 AM) [snapback]429426[/snapback]</div>

Not exact but close. The reason it is close is because the miles (numerator) is large as compared to the gallons (denominator). Which is why I've chosen a small numerator for comparison.

Another point to add is that your units of measurement are not consistent. We know miles divide by gallons is mpg but what is miles*mpg? m²pg?

 

<div class='quotetop'>QUOTE(vincent1449p @ Apr 24 2007, 11:43 PM) [snapback]429612[/snapback]</div>

You win.

 

Well, I'm no math newb, but the way IsrAmeriPrius did it is correct mathematically. The only reason it doesn't work is because it's distance.

Or maybe I've just been to sleep deprived.

 

I know, since I actually had to try both formulas to figure out why his didn't work properly. Like I said though, mathematically he's correct. Like if you took the gas out of the equation. He averaged the numbers properly, but since there is a hidden third factor it doesn't compute out properly.

Either way he was wrong and you were right.

 

Why do you care what your mpg was for a given tank? Are you correcting for odometer inaccuracy? If not, the figure is incorrect anyway. In any case, your lifetime average is much more significant than your mpg for each tankful.

(This from someone who remembered to write down gallons pumped only about half the times I filled the tank, and finally quit record-keeping altogether. And my odometer is off by around 3% anyway, and in the end I don't really care what my exact mileage is, because this is a low-stress car and keeping track and worrying about getting one more mpg out of it is a high-stress activity.)

(I really can't wait for my Xebra to come. That's going to be a really low-stress car, with no gasoline ever and no gas engine to worry about why it didn't shut off when I think it should have.)

 

<div class='quotetop'>QUOTE(Alexstarfire @ Apr 26 2007, 02:21 PM) [snapback]430802[/snapback]</div>

Sorry to interject - but this item is not up to judgment or opinion. This is math - and the equation was just wrong. The formula applied did not keep the units straight.

There is only one way to do this calc: (Mile1 + Mile2) / (Mile1/mpg1 + Mile2/mpg2) = (M1+M2)/(G1+G2) = Total MPG.

 

<div class='quotetop'>QUOTE(daniel @ Apr 28 2007, 09:47 AM) [snapback]431636[/snapback]</div>

daniel, you'd love the mpg estimator on our RX-400h hybrid. It WON'T do per/tank . . . only does lifetime. You can break the cycle by hitting reset, but other wise (like the everready battery bunny) it just keeps going and going and going. I prefer the Prius' per tank, as that way one can moniter the differences between tire pressure, # of passengers, temperature, etc, much easier.

 

leave it to the geeks at PC to post up 17 replies about a math equation

(... i had to. :lol

 

<div class='quotetop'>QUOTE(hill @ Apr 29 2007, 03:06 PM) [snapback]432200[/snapback]</div>

No, I wouldn't. It would tell me that my lifetime mpg is lower than I wish it was. With the Prius I can just mentally more-or-less average those tanks that I feel are worth including, and ignore those winter and short-trip tanks.

OTOH, I'm counting the days until my Xebra comes and I can quit burning gas for in-town driving altogether.