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So you think you know how to calculate MPG?

Discussion in 'Gen 2 Prius Main Forum' started by kirbinster, Aug 10, 2006.

  1. kirbinster

    kirbinster Member

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    People always seem to talk about their average mpg going one way on a route versus two ways and going up hill versus going down hills. See how well you can figure out gas mileage:


    Case-1 Let's say you go up a hill for 2 miles and get 25mpg and then go down a hill (for the same 2 miles with your foot off the gas) and your MFD says you got 99 (or 100) mpg. What was your average gas mileage?











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    If you said 62.5 you are wrong, the correct answer is 50mpg. Let's try a harder one.



    This time you drive 2 miles and the MFD says 40 mpg, you reverse your course (reseting the MFD) and drive 2 miles back to the original destination and since it is slightly down hill you get 60 mpg. What was your average gas mileage for the 4 mile trip?















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    If you said 50mpg, you are wrong -- the correct answer is 48 mpg.



    Last one:

    You take another 4 mile trip, the first two miles are uphill and you get 28mpg while the return trip is slightly more downhill and you get 90 mpg on the display. What was you mileage for the whole trip?











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    Sorry if you said 59 you are way off, it is actually 42.7





    OK, I lied just one more trip.



    This time you took a long trip on the highway, you went 375 miles and the MFD told you that you were getting 52 mpg. You reset the mfd and drive 7 miles uphill to get home and it says you got 25 mpg going up that steep hill. What was your total gas mileage for the 382 mile trip?





















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    If you come up with 50.99 mpg pat yourself on the back!
     
    #1 kirbinster, Aug 10, 2006
  2. eagle33199

    eagle33199 Platinum Member

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    <div class='quotetop'>QUOTE(kirbinster @ Aug 10 2006, 09:20 AM) [snapback]300783[/snapback]</div>
    Actually, it's unknown - you never specified the length of the downhill segment (did you turn around, or was it the other side of the hill that may or may not have been the same length as the first). additionally, 99 simply means that your mileage is above 100 - not necissarily infinite. so the correct answer is some where in the range of 40-50mpg, given a 2 mile long course for the second part (with 40 assuming you got exactly 100 MPG, and 50 assuming you had infinite)

    <div class='quotetop'>QUOTE(kirbinster @ Aug 10 2006, 09:20 AM) [snapback]300783[/snapback]</div>
    For those who don't get it, the correct formulation is: (4 miles) / ((2 miles/40mpg)+(2 miles/60mpg))

    <div class='quotetop'>QUOTE(kirbinster @ Aug 10 2006, 09:20 AM) [snapback]300783[/snapback]</div>
    Once again, correct formulation is: (4 miles) / ((2 miles/28mpg)+(2 miles/90mpg))

    <div class='quotetop'>QUOTE(kirbinster @ Aug 10 2006, 09:20 AM) [snapback]300783[/snapback]</div>
    Finally, (382 miles) / ((375 miles/52mpg)+(7 miles/25mpg))


    the general solution is to take miles divided by mpg for each segment to obtain gallons used in each segment. summing these gallons gives you a resulting total gallons of gas used. taking the total distance divided by the total gallons equals the total MPG
     
    #2 eagle33199, Aug 10, 2006
  3. Drifter

    Drifter Member

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    Always be sure of your math before pointing out problems other people may have.

    In your first problem the answer is only 50mpg if the engine never runs on the way down the hill and you negate the discharge of the battery due to friction. If your milage on the return trip was truly 100mpg the average is 40mpg. If the engine ran but your milage was still above 100mgp for the return trip it would still show 99.9 and your average milage could be anywhere between 40 and 50.

    Oh well, someone beat me to it.
     
    #3 Drifter, Aug 10, 2006
  4. kirbinster

    kirbinster Member

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    Thanks for the sharp Eagle eye, I fixed the first one to include that it is two miles down hill with foot of the gas.
     
    #4 kirbinster, Aug 10, 2006
  5. dreichla

    dreichla New Member

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    How about showing your calculations for each of your problems.

    And - why are you resetting the MFD all the time? The mileage numbers always fluxuate wildly when at the beginning of the reset. It isn't until you are into the tank quite a bit that the mileage numbers start to settle down (I routinely have mileages in the high 70's right after I fill up my tank and drive short distances. It always settles down to a more accurate mileage throughout the rest of the tank of gas.)

    Why not just let the display tell you what the cumulative mileage is for both legs of the trip? And, then only reset after a fillup.
     
    #5 dreichla, Aug 10, 2006
  6. sl7vk

    sl7vk Member

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    Why not simply divide the distance traveled by the amount of gas you stick in your tank? Seems a lot simpler then all of this? Yes, I know it's a bladder, but do it enough times, and your number will be accurate.
     
    #6 sl7vk, Aug 10, 2006
  7. kimgh

    kimgh Member

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    I thought you were wrong about #1 at first. I figured you meant that 100 mpg was the actual FE. Then I realized that you are assuming it's infinite mpg for the downhill leg (which I hope you'll agree is only an approximation...).

    The actual results are a weighted average, "weighted" by the number of miles driven at a particular mpg.

    Nice exercise!
     
    #7 kimgh, Aug 10, 2006
  8. wstander

    wstander New Member

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    <div class='quotetop'>QUOTE(sl7vk @ Aug 10 2006, 07:55 AM) [snapback]300806[/snapback]</div>

    Orrr,

    Win the lotto, buy a Hummer, and never care again how many MPGs you attain....

    sheeeeeshhhhh


    I get 49.7 lifetime MPG+/-; good enough for me
     
    #8 wstander, Aug 10, 2006
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