Huge increase in mpg home today!

Discussion in 'Gen 2 Prius Fuel Economy' started by rustystew, May 3, 2011.

  1. rustystew

    rustystew New Member

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    I was so excited to see this on my screen when I got home I just had to share!
    [​IMG]
    Keep in mind I just started driving the prius last tuesday. And prior to this my best was 64.3 mpg. Which was yesterday as a matter of fact. And the first time I tried the new route I also took today. Yesterdays 64.3 mpg was only a 0.5 mpg improvement over my previous best.

    The route change is a pretty significant change in that rather than take the interstate which is 75mph, I took the access road which has a max speed limit of 50 most of the way. The distance is near identical because it runs along side it. And as a matter of fact may be a little shorter (1-2 miles? really not significant).

    As some of you may know I've got a tach I want to install and I've got a discussion going that I'm hoping will give me some ranges to work with to even further improve my mpg without having to purchase a scangauge for $150-$170. A $30 tach is more my price range.

    However, through these discussions I feel like I'm getting an idea of the different modes the car works in even if I don't have the necessary equipment to tell me definitively. And oddly enough, I am able to feel them as I'm driving. They are very subtle shifts but noticeable.

    Keep in mind also that this trip is largely downhill, but at least I'm no longer competing so heavily against wind resistance. Oh, I guess a big factor could be that the car was very warmed up prior as well. I'm still not entirely sure how big an effect that has. But I suppose with these results it could be fairly significant.

    I guess the real test will be driving back up the hill
     
  2. macmaster05

    macmaster05 Senor Member

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    Hey Rusty, how bizarre I got strangely phenomenal mileage on my way home today too! Usually I can't break the 50 mpg barrier on the freeway on my ride back, but today it was all in the 55-60mpg range! New summer mix gas? Warmer weather, slower traffic? I dont know, but I love it.
     
  3. mikewithaprius

    mikewithaprius New Member

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    Post the uphill when you have it :D

    Nice ride!
     
  4. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I too got great MPG today. I scored 59.8 MPG on a 45 miles commute.
     
  5. Hal W

    Hal W New Member

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    Post the two way average! Hal
     
  6. fuzzy1

    fuzzy1 Senior Member

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    Keep in mind that elevation change is a major factor in Prius MPG.

    My rule of thumb is 10,000 feet being equivalent to a gallon of gas. Over a 20 mile route that would average 64 mpg round trip, a 400 foot elevation change would produce about 57 mpg uphill, and 73 mpg downhill, provided the hills don't require friction or compression braking.
     
  7. rustystew

    rustystew New Member

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    Man if I could get 57 back uphill, I would be super excited. Right now my best trip in has been 39.2 mpg I believe. Still not horrible but definitely not 57! I'll let you guys know after things wind down at the office tomorrow.
     
  8. uart

    uart Senior Member

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    Yep I've got a few routes where I get 100+ MPG one way and then about 30 MPG coming home. Big changes in elevation.

    Technical tip about two way averaging. If you reset your mileage so that you measure the MPG separately each way then you cant just average the two separate MPG numbers, it doesn't work. If both trips are the same length however then it does work to take what's called the "harmonic mean". That's where you average the reciprocals and then take the reciprocal of the final answer.

    For example, 100 MPG one way and 30 MPG back doesn't give 65 MPG average. It does however give 2/(1/100 + 1/30) = 46 MPG.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    This suggests a much greater elevation change. I'll make a first guess of 1200 feet, but reserve my right to change it once you post round trip and new uphill figures.
     
  10. rustystew

    rustystew New Member

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    Wow, that's probably pretty close. Wikied the elevation of galisteo, NM (home) 6,056ft and Santa Fe, NM (work) 7,260ft. Probably couldn't have made a better guess!

    Interesting, I had no idea. I had just been adding the two together and dividing by two. Maybe I won't reset In the morning then. But I do like knowing my one-way mpg. At least I have an accurate formula now!
     
  11. SageBrush

    SageBrush Senior Member

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    Trip from my home to the ~ Rio Grande river: 188 mpg. Back home: 35.5 mpg.
    Average: About 60 mpg over 15 miles. Not impressive, but 15 miles is still a short enough trip to clearly show the the warm-up phase.

    I was driving slow and had the emergency blinkers on. My wife joked that carrying a large plate of glass and driving on pins and needles is the best way to get high MPG ;)
     
  12. rustystew

    rustystew New Member

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    Well Guys here it is!
    Again notably higher than any past attempts, and I hit more stoplights than usual.
    [​IMG]

    So with the "harmonious average" of the two we get an awesome 55.1 mpg!
     
  13. SageBrush

    SageBrush Senior Member

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    ^^ Very nice!

    It is pretty easy to estimate one-way fuel economy with altitude changes:
    The change in elevation is m*g*h in joules, so in your case
    1400 kg * 9.8 * 380 meters = 5213600 joules.
    Plug in '5213600 joules in kilowatt*hours' to google (or divide by 3.6 million) to get 1.45 kwh.
    Divide the 11.75* into the kwh to get gallons of petrol spent on the climb (or saved in the descent). For this trip 1.45/11.75 = 0.123404255 gallons

    Almost there ...
    You consumed 21/44.3 gallons of fuel = 0.474040632 gallons
    0.123404255 gallons was for climbing, so 0.474 - 0.1234 = 0.3506 gallons on level ground.
    MPG = 21/0.3506 = 59.8 MPG! Figure +/- 5% inaccuracy.

    * 34.4 kwh of energy in one gallon of petrol, and assumed 34% engine efficiency
     
  14. BAllanJ

    BAllanJ Active Member

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    I can just do the obvious averaging but that's because my mfd reports consumption not mileage... in litres per hundred kilometers. So it's pre-reciprocalled!

    (although I recently changed tires to the ep100s and now my speedometer is reading off by 1%... so now I expect it's reporting in litres per 99 km)
     
  15. fuzzy1

    fuzzy1 Senior Member

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    For those unwilling to go through the math, this essentially equates 1 gallon of gas to 10,115 feet of elevation change.

    My rule of thumb has a similar derivation.
     
  16. rustystew

    rustystew New Member

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    Thanks for all the info guys. Nifty stuff
     
  17. usbseawolf2000

    usbseawolf2000 HSD PhD

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    This is the round trip commute including two short trips (lunch and back) and a drive-thru (dinner).

    The car has 143k miles and it still drives like brand new. :-D

    [​IMG]
     

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