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  1. #1

    is your GPS accurate for MPH?????

    I recently bought the "Garmin Etrex Legend H" and i already have major doubts. This unit only cost $120 and i figured it would be great for a speedo if for nothing else.

    I compared this GPS's speed against my truck's speedo and it's apparently registers 3-4mph faster than the truck (stock rims/tires on '08 Tacoma). With the truck at 50mph, the GPS reads 53-54.
    Also, I noticed that when i climbed roughly 30-40ft in elevation along the hwy, the GPS would only change to an additional 10ft or so when i came to a stop at the top of a hill.
    I have enough mods to squeeze out mid 70's all day with my ski. But after using this GPS last week - after the test with the truck, im BAREY squeezing out 71mph! So im all out in left field as far as what the ski is running and my trust in this GPS.

    So whats the deal....who's GPS is dead-accurate and trustworthy?!?!?!?!?? I always thought most any GPS was hella accurate!!
    What are you guys using and do you REALLY know if it's dead-accurate???

    Im dying to know who's using the best GPS for speed because Id like to send this one back and get that one!

    Thanks for all input!


  2. #2
    Booner76's Avatar
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    I have a Garmin GPSMAP 76 (came with 04 GTX LTD). I have checked it with my Garmin 255 (nuvi) I use in my car.
    It's right on.

    Cheers

  3. #3
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    Quote Originally Posted by blkturbo! View Post
    I recently bought the "Garmin Etrex Legend H" and i already have major doubts. This unit only cost $120 and i figured it would be great for a speedo if for nothing else.

    I compared this GPS's speed against my truck's speedo and it's apparently registers 3-4mph faster than the truck (stock rims/tires on '08 Tacoma). With the truck at 50mph, the GPS reads 53-54.
    Also, I noticed that when i climbed roughly 30-40ft in elevation along the hwy, the GPS would only change to an additional 10ft or so when i came to a stop at the top of a hill.
    I have enough mods to squeeze out mid 70's all day with my ski. But after using this GPS last week - after the test with the truck, im BAREY squeezing out 71mph! So im all out in left field as far as what the ski is running and my trust in this GPS.

    So whats the deal....who's GPS is dead-accurate and trustworthy?!?!?!?!?? I always thought most any GPS was hella accurate!!
    What are you guys using and do you REALLY know if it's dead-accurate???

    Im dying to know who's using the best GPS for speed because Id like to send this one back and get that one!

    Thanks for all input!
    I have the same GPS, it is accurate to withing about 20 feet. Take it out with a measuring tape, reset the distance to zero, and start walking, and see for yourself. It can use up to 8 satellites, but needs only 3 to triangulate your position.
    As for speed, it is within 1/10th of a mph. However, when measure your speed on a moving surface(water/lake), remember that there may be some current/flow, that will be added/subtracted from your speed. The GPS is measuring your speed across the Earth. Same with distance on the water. If you have a waypoint up river, for example, and it says it's only 5 miles away....that is 5 miles in a straight line/as the crow flies, not river/lake miles, where you may need to make turns.

    Some car speed-o-meters are off, some by as much as 10%, straight from the manufacturer. Go by one of the radar signs and see for yourself.

    So, the reason you may only be "squeezing out" 71mph on your ski has more to do with the weather we've had lately, and less to do with the GPS. It has been humid, and hot as hell here lately(Lake Havasu), so you are density may be much less, than the last time you tested.
    You can gain, and lose 2 to 3 MPH, on the same day, just due to changes in air density.

  4. #4
    Wow....so this all means that none of us are ever going to know 100% just how fast we are going between mods??
    Thanks for your detailed input though!


    Anyone else ahve anything to add?

  5. #5
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    Quote Originally Posted by blkturbo! View Post
    Wow....so this all means that none of us are ever going to know 100% just how fast we are going between mods??
    Thanks for your detailed input though!


    Anyone else ahve anything to add?
    It depends on the mods, and how much the air changes between when you install the mods, and the last time you tested.
    For example; if your fastest speed was recorded in December, and now your dumping a bunch of money on mods, don't expect to see the huge increases. Your in Vegas.....it's hot, and about 2500? feet above sea level. So, i the winter, you ski may feel the altitude is only around 500 feet. But in the summer, and density altitude may be more like 5000 feet. Welcome to the Mohave Desert.

    The only way to know exactly what difference the mod did to your speed, is to test under the exact same atmospheric conditions. That's why most companies do their testing in the winter. It's usually around 59 degree's here, and the weather is usually consistent from day to day.

    My question to you would be this; when, and with what GPS did you get the 75.9mph?? What speed are you getting now, and what mods have you changed to try and increase your speed??

    Also, you get to the point where it really doesn't matter; it's just a number. Some mods only improve acceleration, or handling. I would be happy with 75mph all day long, in the heat. But, the best I can squeeze out of my stock 08 RXT-X, with full fuel, is 67.9mph right now. That's at 450 feet above sea level, and the temp is around 100 degree's. In the winter, I can get close to 69.5mph. So, not a really big difference....but, it's a stock boat.

  6. #6
    YoYamma's Avatar
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    In my experience, the $100 - $200 Garmins are not super accurate for speed but they are okay. I recently tested one and it was a little off (slow). We have also been testing some of the speed apps for the iPhone. We have a member named SurfRider here on the forum who developed a free app called "Max Speed." You can get in the App Store now. I compared it to my vehicle speedo and also my $1,000 Garmin on some water tests last week and it appears very accurate.

  7. #7
    Quote Originally Posted by iflyems View Post
    It depends on the mods, and how much the air changes between when you install the mods, and the last time you tested.
    For example; if your fastest speed was recorded in December, and now your dumping a bunch of money on mods, don't expect to see the huge increases. Your in Vegas.....it's hot, and about 2500? feet above sea level. So, i the winter, you ski may feel the altitude is only around 500 feet. But in the summer, and density altitude may be more like 5000 feet. Welcome to the Mohave Desert.

    The only way to know exactly what difference the mod did to your speed, is to test under the exact same atmospheric conditions. That's why most companies do their testing in the winter. It's usually around 59 degree's here, and the weather is usually consistent from day to day.

    My question to you would be this; when, and with what GPS did you get the 75.9mph?? What speed are you getting now, and what mods have you changed to try and increase your speed??

    Also, you get to the point where it really doesn't matter; it's just a number. Some mods only improve acceleration, or handling. I would be happy with 75mph all day long, in the heat. But, the best I can squeeze out of my stock 08 RXT-X, with full fuel, is 67.9mph right now. That's at 450 feet above sea level, and the temp is around 100 degree's. In the winter, I can get close to 69.5mph. So, not a really big difference....but, it's a stock boat.

    Thanks everyone so far...

    To answer your questions, Im using the identical GPS for speeds. It was Feb when i pulled 75.9.....last week i barely hit 71. No addition or subtraction of mods.
    Of course thats two different seasons, but what raised an eyebrow was the large difference between car and jetski on the GPS. I thought "movement" was movement, and that most any decent GPS would be spot-on. But with this one being 3-5mph off from a car (just my truck so far - factory wheels etc) and apparently sketchy on elevation too, im very skeptical and disappointed. Im wondering if Amazon.com would take this GPS back and sell me a different one
    Plus, im all familiar with heat soaking and different capabilities between such factors, but damn, 5mph on the ski with no difference in mods - while still having somewhat cool water too - not blazing hot or very warm.

  8. #8
    Vern's Avatar
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    I have used multiple GPS, including the Garmin Etrex ... all were very close to the same speeds. The Etrex I have now always measures almost exactly the same as my Grand Prix speedo, but we found out my girlfriend's BMW is always 5 mph low at 80 mph ... so from my experience, the GPS is right, and usually very close. Car speedos vary a bit, even with stock gearing and tires. I would trust the GPS over any speedo every time.

  9. #9
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    I agree, you have to trust the GPS over a speedo; especially boat and PWC speedos are notoriously inaccurate devices. By the way; I have to say, the GPS on the new iPhone 4 is amazing. We were out on the lake a couple of weeks ago and it got dark on us; we had to find a small river to navigate back home so I pulled up the Google map on the iPhone, and it gave us a beautiful, high detail daytime satellite view of the entire lake with our boat right in the middle. It's amazing how the smart phones have turned into into location devices; you just have to remember to use them; I still don't think of my phone as a tool like that.

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