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2005 Prius at 65 mph, MPG?

Discussion in 'Gen II Prius Fuel Economy' started by Dogthree, Jan 18, 2012.

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  1. Dogthree

    Dogthree New Member

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    Hello everyone,

    I am investigating a "device" for saving fuel to see if it works.

    My guess is no.

    I want to ask the following... if you take a 2005 Prius, put on a slightly freer-flowing exhaust, and drive it at 65 mph on the cruise control for one hour (65) miles, how many gallons of gas would you expect it to have burned?

    There were three persons on board, the ride was quite bumpy (over-inflated tires maybe?) and as I alluded to, the exhaust was unnaturally loud. Total fuel consumption I think was 1.06 US gallons (although the driver claimed 0.9 gallons because he spilled some on the ground). I think that comes to 59 mpg.

    A/C was off, windows up. Temp 55 F, 20% humidity, almost flat course. (Southern California)

    Now I rented a Gen III Prius myself the next day and drove the exact same course in the exact same way, as close as I could get it. Temp outside was 59 F, and I was alone in the car. I was somewhat surprised to get only 58 mpg. I had over-inflated the tires to 54 psi.

    Are there any simple things (over-inflated tires, catalytic converter delete, thermostat hacks, etc.) that could be done to a Gen-II Prius to bring its gas mileage to 60-65 mpg at a constant 65 mph? To 70 mpg?
  2. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    Bob Wilson used to have MPG graphs for Gen 2 and Gen 3, (which I can't find) they were very close at 65

    here is his Gen 3 mileage notice that 65 is dip.
    [​IMG]
  3. fuzzy1

    fuzzy1 Senior Member

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    I can't speak for a 2005, but have some comments that will apply to any Prius.

    "Almost flat". How much elevation change from between the start and end points? Even 150 feet of descent over that distance is worth roughly a 1 mpg boost.

    High tire pressure helps, many of us run in the 40s. A very few hypermilers on a different forum run much higher. Note also any odd tire size, as a slightly small tire will significantly boost apparent mpg, even thought the real number is unchanged. This is a double whammy, from both speed error and distance error. Worn tires will also read higher, both from easier flexing and from smaller tread diameter.

    How was fuel measured? The 2005 has a fuel bladder that has notoriously inconsistent refill behavior, enough that many consecutive full tanks need to be added together to get accurate mpg numbers. The built in mpg gauge on the dashboard is much more accurate for shorter trips.

    Pure gas, or E10 ethanol? This ought to make a 3% difference.

    Cold start, or was the car pre-warmed? This probably makes a 2 mpg difference over an hour trip.

    I'm sure there are plenty more things to watch.

    Was there any tailwind? Even a small breeze will help.
  4. cwerdna

    cwerdna Senior Member

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    How are you measuring 1.06 or 0.9 gallons? If you're doing what I guess you're doing, the fill was too short/small and there's a huge margin of error.

    As for the Gen 3 case, why are you surprised?

    Were the trips made going in both directions over the same route, to help cancel out the effects of elevation change and wind?
  5. Dogthree

    Dogthree New Member

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    Yes I am aware there's a huge margin of error. As you probably guessed we filled the tank to the "lip" from a fuel pump (same one, with the car in the exact same spot, and at least we didn't use the automatic shutoff). Total elevation gain was zero, route started and ended in the same exact place. Engine was warmed up. Come to think of it the Gen III probably wasn't as well warmed up as the Gen II.

    I was surprised by two things: 1. the two cars were so close together even though the measurement method was so apparently clumsy. 2. the Gen III was a tad worse. The EPA #s for the Gen III are better.

    Wind was essentially zero, and it was a course that, as I mentioned, returned to the same spot.

    Since it's California I'm assuming it's E10. Don't think we can buy anything else anymore. It was from a regular Chevron pump.

    What I'm curious about is how big a difference, roughly, things like overinflated tires, cat delete, thermostat change could make. Things that could easily have been done to the Gen II car but that I didn't get a chance to check for.

    I did notice the issues with the Gen II fuel bladder. That's why the range is 0.9-1.1
    gallons for it, not just a number. The Gen III really did seem to fill to precisely the same
    level. No, not the fuel pump shut off but actually filled to the lip of the filler tube.

    I didn't choose this method for calculating gas mileage---I certainly wouldn't have done it this way. In fact I would probably have suggested drag racing to figure out whether the device worked at all. I think the people who came
    up with it are scammy and are trying to trick people into believing that their device
    is magical when in fact the results are unremarkable. They seem 5 mpg better than the graph above but that could easily be explained by the crude measurement method.

    I'm very grateful for the information I'm getting here!
  6. cwerdna

    cwerdna Senior Member

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    I doubt that whatever device it is makes any difference when it comes to FE.

    As for huge margin of error, even on much larger fills, I and others on the Gen 2 can find the manual mpg calculations are way higher than the MFD (multifunction display) on one tank and much lower on the next, even on "fills" of 6+ gallons each, partly due to the bladder, guess gauge and complex fuel tank assembly. To do fills of only ~1 gallon really means a huge margin of error. Unless you're going to install something in the fuel system to measure usage/flow, you're better off trusting the MFD in the Gen 2. It's pretty accurate.

    The Gen 3's trip computer supposedly reads a bit high, so you'll likely need to take off a few mpg.

    You might find these insightful: http://priuschat.com/forums/other-c...uth-about-epa-city-highway-mpg-estimates.html and http://priuschat.com/forums/other-cars/94001-gas-gauge-says-full-but-thats-not-quite-true-npr.html.

    Unfortunately, the EPA ratings on Compare Cars Side-by-Side for the 2nd gen were most likely obtained by adjusting the pre-MY 08 EPA numbers via formula vs. running the new test procedure (w/3 extra tests) that the Gen 3 most likely went thru. We may never know the numbers the Gen 2 achieves on the newer tests.

    Most fuel-efficient cars is what CR got in their testing on Gen 3. http://web.archive.org/web/20081231...-advice/most-fuelefficient-cars-206/index.htm is for Gen 2. Last page of http://www.consumersunion.org/Oct_CR_Fuel_Economy.pdf describes their method, sorta.
  7. Dogthree

    Dogthree New Member

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    Aha the Gen II are just adjusted from the old EPA numbers.. what do you know...

    I'm really just trying to establish that I haven't seen anything worth remembering, which seems about right :)
  8. uart

    uart Senior Member

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    You seem to be pretty "hush hush" on the nature of the device. Is it a secret? Are they trying to get you to invest in it?
  9. fuzzy1

    fuzzy1 Senior Member

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    We haven't established that at all. It is completely possible that they did show you something worth remembering.

    The problem is that this is still within simple spin range, should they choose something less than full disclosure and honesty or are not fully informed about what is possible without magic devices. It is up to them to demonstrate that this is showing a real improvement from their magic device, and they have not yet done so.

    Question: At what elevation was this test? Higher elevations will produce better results, though hybrids benefit by a smaller fraction than do non-hybrids. (All cars face less air drag in the thinner air of high elevation. Non-hybrids also get some part-throttle engine efficiency improvements, while most hybrids already get even more of the same thing independent of altitude.)

    Suggestion: Next time, take a GPS to crosscheck both the speedometer and the trip meter. If the promoters are being honest, the speedo should be lying by 1-2 mph (this is an industry norm and a legal requirement in some areas), while the odo should be accurate to a fraction of a percent. If the car's measurements are more optimistic than that, somebody has fudged something, such as the tires.
  10. angelman

    angelman New Member

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    I just purchased a 2007 with 71k and my first fill up today was 9.1gal. The mfd showed 46.5mpg and my actual calculations: 483miles divided by 9.1 gave me 53.07mpg. The fuel gauge had just changed to one square left, thats when I decided to get fuel. I still had 1.8gal left in the tank. Not bad for a first fill up. It should do better when it gets warmer out. I usually never drive faster than 60mph. At 65 noticed a big drop in mpg. Great forum.:)
  11. F8L

    F8L Protecting Habitat & AG Lands

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    How do you know you still had 1.8 gallons left in the tank? :confused:

    You do realize that you have a bladder-style fuel tank so the actual capacity changes with temperature right? The capacity is not always 11.9 gallons and in some cars it may never be. :)

    Please spend some time doing searches on the topic of gas tank capacity. You'll quickly find out why it is not a good idea to try and rationalize how much fuel you have left and just "listen" to the car when it tells you to add fuel. The best way to avoid running out of fuel is to fill up when the gauge reaches two pips (bars). I'd hate to hear you ran out of fuel in a dangerous area.
  12. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    The gas tank is larger when it is warm, and smaller when it is cold.

    So I have pumped over 11 gallons in summer but often can't fill it over 9 gallons in winter. Even morning to afternoon is a different size tank. Be very careful making any assumption about gas you have left. I have run out, if you play below one pip, you will too, just a matter of when.
    Alone with no one to strand, I get gas at one pip.
    With years of time to hear about it, I get gas at two pips when my wife is in the car.
    With your mother in law in car, get gas at three pips.
  13. tv4fish

    tv4fish Member

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    Definitely "sage" advice. LOL :)
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