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  1. #1
    bowsniper's Avatar
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    Why are MFD's wrong. aka. "Dream-O-Meter"

    Why are MFD's wrong when compared to a GPS? Think about it for a sec. Water gets pushed into the air tube. The amount of push or force is measured and displayed in real time as it varies. The water can't go any faster into it as its working so you will never has a wrong reading as far as top speed reading goes...unless the gauges calibration is off. Which I think would be digitally stored info. The more pressure the higher the reading.

    Now other factors come into play too. The direction of the tip is important. It screws into the hull and must be straight looking forward. (Obviously). Second, good connections on both ends...ie..gauge and pitot. Third. The line is free of debris. Fourth, is the Length of the line important? The longer the hose, the less pressure (force) or reading on the gauge? Is that true?
    .
    Can they be calibrated? How accurate is GPS now a days? is it dead on? Whats the difference between the two in MPH?

    Your thoughts, experiences from hiking, motorcycle, drag strip, jetsking? What do you think?

    If gps is that accurate, why don't cars just use that for a speedometer? Why use a car speedometer connected to the trans?


  2. #2
    Miglater's Avatar
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    I would guess they are off because they read from the pump tunnel and the pump doesn't process the water as fast as its loaded.

  3. #3
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    They are actually very accurate, except when it comes to top speed on long WOT runs. I have my GPS right next to the MFD and they are within 1 MPH at all speeds, except when running at WOT for long runs, then the MFD shows higher than actual.

  4. #4

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    if there accurate then my 97 sl 1050 did 74mph out of the box ??? ,like you say on long runs there off ALOT !!!!!!!!!!!!!!

  5. #5
    RLACEMAN's Avatar
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    MFDs were never meant to be very accurate, just a general speed reading. Probably 2 reasons, first is the very design of using a pitot for speed, the water pressures are always changing under the ski, such as small waves, turning changes the angle of the pickup too, I always thought that was a bad location for a pitot, it it were on the back of the hull like a boat, it would be more consistent, & a little more accurate, also the Polaris mfds may be too sensitive to small changes in pressures, my 750 at wide open reads anywhere between 51 & 57 but its top speed is 51(gps). My Kawi uses a magnetic wheel on the back of the hull which is more consistent, but not much more accurate & on both brands it seems once you get past the low 50's they start reading several mph fast. Highway driven vehicles need to be accurate by law, also so all the speed related computer functions work right, & its easier to make a 'geared' vehicle read correctly, even my late model snowmobile is accurate, on a blast across the lake the speed reads only 2 mph faster than the gps, & I'm sure there is a little slippage on the ice & snow.

  6. #6

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    Hence the name Dream-o-Meter

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Arrow How does a GPS measure speed? How accurate is this?

    GPS do not measure speed.

    GPS units calculate position, perhaps several times per second. The accuracy of each GPS position 'fix' depends on the number of GPS satellites that your unit is currently able to 'see' in the sky and how strong and consistent those satellite signals are.

    If the GPS view of the sky is partially blocked by your body, parts of the watercraft, even heavy cloud cover, then the GPS satellite signals will be weaker and less accurate.

    So the GPS internally has a frequently updated sequence of recent positions and the exact, precise time each position was recorded. The GPS then does some math to calculate the distance between each successive pairs of locations. It knows how much time passed between one location fix and the next, and how far apart those two positions were.

    Since speed is simply distance divided by time (miles per hour) your GPS can then display the calculated speed. This is the instantaneous (current) speed you see on your GPS display.

    There are some limitations with GPS speed calculating.

    If the accuracy of each position fix is poor then the speed calculation will necessarily be inaccurate. When using a GPS while standing still sometimes you will see your current position wandering around on the screen, even though you are not moving. This is position fix inaccuracy.

    A GPS will average these position errors over time so the average speed display will be fairly accurate. However, from instant to instant, the calculated GPS speed can be quite wrong at times. This is how you can get crazy fast 'recorded peak' speed numbers on a GPS. If you watch your GPS while flying across a large expanse of smooth water you will sometimes see the speed reading jump up or down for a moment, then return to more normal numbers.

    Some GPS units are much better than others about excluding inconsistent short term speed numbers. Others will happily report and record whatever crazy number was the highest, even if it is unlikely to be real.

    If you are accelerating or slowing down, the GPS does not directly know this. It just keeps on calculating positions and the distance between them and displaying what constant speed would be necessary to cover that distance in that time.

    This is why when you use a GPS to monitor speed while you accelerate up to top speed, the GPS speed reading will lag for a second or two after the hull tops out, eventually settling down to display a stable continuous speed reading. This speed display lag is one reason GPS are not used in regular road vehicles.

    Sea Doo does use GPS speedometers in some models. I have not ridden them and don't know how quickly the speedometer reacts to actual speed changes. It expect it is possible to use software to sense acceleration and anticipate speed changes for the speedometer display, thereby reducing the apparent accuracy lag.

  8. #8
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    Thanks for GPS breakdown, very good information

  9. #9
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    My MFDs have all been within a 1.5 mph 90% of the time. Once in a while I will get a crazy reading...But then again my GPS showed a top speed of 225 MPH on a low 80mph run at dale hollow lake years ago.

  10. #10
    bowsniper's Avatar
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    Well if the mfd is only 1.5 mph off....my pro did 68.7 mph . . I doubt it. Even with a 1.5 mph reduction.....that would be 67.2 mph and we all know that's not either.. a stock pro does about 61-63. Thats Top speed with stock pipes , heads, compression.ete.
    so that means its more like 4 mph off. i guess a side by side comparison is in order in summer.

    Has anyone else done a side by side and see what it is? Lavey T. stated his numbers @ 90%. Can anyone else confirm thier numbers for comparison? I have seen 67.4 also and lots of 63.4's. Im inclined to believe the 63.4 is more realistic.

    Thanks for the input everyone. Now on my 95 slt the mfd says 48-50 at wot. Now that's really close to being correct. My old 750al said 50. and my slx said 68.7 and lots of 63.4's. And that's pretty close to correct too. Hmm the story continues. I would guess its off at least 2 mph. If not more...

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