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  1. #1
    LaveyT's Avatar
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    Cool GPS Numbers, what are yours ?

    Hey All,
    I got a chance to check the top speed on both of my ski's. I was a little suprised. The 1995 SLX had a "Flash" reading of 56.9 mph but I was never able to get it that high again. 55.9 mph was the "Repeatable" best top end.
    The 1996 SL780 had a top end of 53.2 mph. I didnt think it would be that close to the SLX. 52.5 mph was the best "Repeatable" top end.

    are these numbers typical for stock machines? I think elevation is about 600+ feet. The air was good 70 degrees or so.I GPS my old SL750 @ 50.4 some years ago.
    Last edited by LaveyT; 04-12-2010 at 08:41 AM. Reason: sp


  2. #2
    Race, wreck, repeat delagem's Avatar
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    My GPS readings for all my skis are in my signature.

    GPS isn't really the most accurate way to measure speed. I use it a LOT at work, and it's surprising how much it jumps around. It jumps every time the unit sees a different satellite and decides to switch.

    I've also read that having a GPS bouncing around in a storage compartment will give higher speeds than mounted rigidly to the bars. Never confirmed this though.

    The key to accurate GPS is Differential GPS. That's what we have on the commercial ships for high accuracy in coastal waters. I honestly don't know if any handhelds are using differential, but I doubt it.

    I've been meaning to buy a RADAR unit for more accuracy.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by delagem View Post
    ...it's surprising how much it jumps around. It jumps every time the unit sees a different satellite and decides to switch.

    I've also read that having a GPS bouncing around in a storage compartment will give higher speeds than mounted rigidly to the bars. Never confirmed this though...
    A GPS unit calculates speed based on the time between each position locate and the next.

    When the satellite signals are weak, or the GPS is bouncing around (or both), there will be times when the positions themselves are rather inaccurate. The trick is to NOT reply on the 'maximum speed' facility in the GPS.

    Mount it somewhere where you can see it clearly, AND the GPS has a good view of the sky above and around. For accuracy, the GPS needs to see more than three satellites, and those satellites need to be around 45 degrees up from the horizon, give or take.

    A decent GPS will track about a dozen satellites once it has had a few minutes to get a signal lock on them all. A good GPS will have a screen that shows you how many satellites it is tracking, and how strong the signals are.

    Outside on the water, the signals should be many and string. If not, get another GPS unit.

    GPS signals from the satellite will NOT go through your body. If you mount the GPS down low in front of you, your body will act as a radio shield, and reduce the GPS accuracy.

    While riding, monitor the displayed current speed. Even if the speed numbers are jumping around a little, you will quickly see what the highest consistent speed number is.

    All the GPS maximum speed function does it record the highest number calculated for current speed. Since we know that the current speed number can be a bit jumpy, that makes the 'highest speed' function somewhat inaccurate.

    Summary
    Weak GPS satellite signals will result in less accurate speed numbers.
    Mount the GPS firmly where it will see the maximum amount of sky, with nothing in the way, and will not be flopping around.

    Here is an example of what can happen when you don't give the GPS a good sky view;
    31.4 MPH.

  4. #4
    LaveyT's Avatar
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    Thanks for the input.

    I was just holding the GPS in my hand with the cord around the bars. I dont plan to obsess about speed as it makes me unhappy. I had my speed boat up to 94.7 mph and was never happy. then I would change something and try to go faster yet. It ended up being more about the number on the GPS than boating. Now it goes 80-85 depending on the prop and conditions.
    I suppose 52mph (6400 rpm) for a SL780 and 55mph (6650 rpm) for a SLX is about what I should expect (normal) ?

    I did have a "Flash reading" of 225 MPH @ Dale Hollow on one run with the Lavey. I kind of took that as a sign I should stop obsessing and just go boating

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    The SL 780 and the SLX 780 use the same engine, just slight differences between them. FA's, intake grate, maybe long jet pump depending on year, and impeller. Not that much different between the 2, so a couple MPH difference sounds about right.

  6. #6

  7. #7
    Plrs X45's Avatar
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    Quote Originally Posted by xlint89 View Post
    The SL 780 and the SLX 780 use the same engine, just slight differences between them. FA's, intake grate, maybe long jet pump depending on year, and impeller. Not that much different between the 2, so a couple MPH difference sounds about right.
    Also, side by side 2mph is a big difference. I would love to get 2mph more out of my genesis. Those GPS readings are pretty accurate for those boats.

  8. #8
    LaveyT's Avatar
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    Thanks

    Quote Originally Posted by xlint89 View Post
    The SL 780 and the SLX 780 use the same engine, just slight differences between them. FA's, intake grate, maybe long jet pump depending on year, and impeller. Not that much different between the 2, so a couple MPH difference sounds about right.
    Thanks to all for your replys. The handeling difference is amazing between the two ski's. They are both a blast in there own way. The Sl780 is great for flipin and floping around spin outs ect.
    I havent found the breaking (hooking) point on the SLX yet. Im not sure I want to it turns so hard without sliding it's gotta hurt when it hooks. It takes no real talent to get it to go. with the SL its all about throwing your weight around.
    the SLX Speedometer was reading about 3 MPH high. the SL780 was within a few tenths on the top end.
    Both speedometers were very close to the GPS (Tenths) at 15-30 mph.
    Damn it feels good to ride again!

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    What kind of RPM were you turning out of the 2 skis?

  10. #10
    I'D RATHER SWEAT THAN SHIVER hyoctane23's Avatar
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    I have only GPS'd my '98 Pro 785. Seems the dream-o-meter is "fairly" accurate. I would have expected it to be off a lot more.


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