GPS wrong?
Moderators: Redneck_Randy, badmoonrising, lakerunner
GPS wrong?
just wondering, whats the chance that a gps is wrong on the speed?? in my truck it bounces back and forth about 3 mph but on the boat today it was steady.
reason i ask is that i have a antique depth finder with spedo wheel on it and it reads 13 wot but the gps says 16 wot
maybe the old 50 has more life in it than i thought
reason i ask is that i have a antique depth finder with spedo wheel on it and it reads 13 wot but the gps says 16 wot
maybe the old 50 has more life in it than i thought
1982 Sylvan 24' Toon
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
- badmoonrising
- Site Admin
- Posts: 10066
- Joined: Fri Mar 30, 2007 7:04 pm
- Location: Chesapeake City, Maryland
Re: GPS wrong?
My GPS is dead on accurate on the boat.
Ed, Cheryl, Ethan and Aspen.
2013 Sun Tracker Party Barge 22 w/90 HP Mercury, "Hellrhighwater 2"
2014 E-350 Extended XLT.
Chesapeake City ,MD
2013 Sun Tracker Party Barge 22 w/90 HP Mercury, "Hellrhighwater 2"
2014 E-350 Extended XLT.
Chesapeake City ,MD
Re: GPS wrong?
It your truck make sure the GPS isn't obstructed by the cab it will shield the signal and cause inaccuracy.
2013 South Bay 442CR STT
150 HP Mercury 4 Stroke
150 HP Mercury 4 Stroke
Re: GPS wrong?
thanks guys il have to put the gps up on the dash and see if it stays steady
1982 Sylvan 24' Toon
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
Re: GPS wrong?
GPS units usually display how accurate they are to the foot range based on the current signal. My Garmin Nuvi gets to about 16 ft accuracy with a strong signal. Might check the signal strength in your truck vs your boat.
Re: GPS wrong?
I don't know where you live. But if you have hills to climb then you do get a little different reading. It's hard to hold a truck or car at one speed for a long time even with cruse control set. But if you let it go for a long while 40, 50 miles it will read the correct speed. Most time your boat wont climb up hills or waves for 1/2 or 6 miles at a time. Your gps is right your old one is wrong.
2001 22' Tracker Regency (sold)
2001 Mercury 90 HP (sold)
Elm Grove, La
Lake Bistineau
2001 Mercury 90 HP (sold)
Elm Grove, La
Lake Bistineau
- Alohatoon1
- Posts: 48
- Joined: Mon Jul 13, 2009 2:01 pm
Re: GPS wrong?
Don't forget that if you are going 13mph in a 3mph current your GPS is going to give you true ground speed of 16mph.
Your speedo is still going to give you 13mph, true relative speed.
Your speedo is still going to give you 13mph, true relative speed.
Please visit my website if you get a chance, better yet submit a couple of your favorite photos there!
The Boating Photo of the Day
The Boating Photo of the Day
Re: GPS wrong?
Alohatoon1 wrote:Don't forget that if you are going 13mph in a 3mph current your GPS is going to give you true ground speed of 16mph.
Your speedo is still going to give you 13mph, true relative speed.
HUH never thought of that but we travel on a big lake where there isnt any current (or to be seen) if its calm out you can sit in one spot for hours and not move
thanks yall
1982 Sylvan 24' Toon
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
Re: GPS wrong?
To add a bit to the in a vehicle and hill situation, think about the speed for a minute. The GPS is triagulating from 3 dirrerent points to get your speed. These sattelites don't see hills or valleys, they just see how fast you are moving towards or away from them. So, it retrospect, your car may be going uphill, or downhill at 70mph, but you are actually going less than that to the GPS because you are going up/down a hill, and therefore aren't moving accross the plane ( earth ) as fast as your wheels are actually spinning. Much like the current and speedo situation.
I hope I didn't loose anyone there.
I hope I didn't loose anyone there.
"Nauti Weasel"
2005 Beachcomber Islander 26
2005 Beachcomber Islander 26
Re: GPS wrong?
LOL that makes sence and with me living in the mountains its ALL up or downGXPWeasel wrote:To add a bit to the in a vehicle and hill situation, think about the speed for a minute. The GPS is triagulating from 3 dirrerent points to get your speed. These sattelites don't see hills or valleys, they just see how fast you are moving towards or away from them. So, it retrospect, your car may be going uphill, or downhill at 70mph, but you are actually going less than that to the GPS because you are going up/down a hill, and therefore aren't moving accross the plane ( earth ) as fast as your wheels are actually spinning. Much like the current and speedo situation.
I hope I didn't loose anyone there.
i did get on about 30 miles of flat road the other day and no matter where i put the gps in the truck it still said about 5-6mph slower than the spedo who knows LOL
1982 Sylvan 24' Toon
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
50hp Evenruder
my fishin/camping boat
An unarmed man is a subject.
911: Government sponsored Dial-a-Prayer.
When seconds count....Police are minutes away
- curtiscapk
- Posts: 5401
- Joined: Mon Nov 10, 2008 8:31 am
- Location: OP KS\Hillsdale Lake
Re: GPS wrong?
could be your tire size my daughter got the wrong size tires replaced on her truck (since they were cheaper) and the gps now reads 5 mph slower than speedo when before with the bigger tires the were dead even.
Craig and Paula
"THE FLOATER" rebuild Spring 2013
viewtopic.php?f=2&t=15328
94 Party Barge 24' 115 merc
Turning Point hustler 14 x 13 prop
22mph gps 3 people
12 F150
Overland Park Ks
Hillsdale Lake, KS
"THE FLOATER" rebuild Spring 2013
viewtopic.php?f=2&t=15328
94 Party Barge 24' 115 merc
Turning Point hustler 14 x 13 prop
22mph gps 3 people
12 F150
Overland Park Ks
Hillsdale Lake, KS
Re: GPS wrong?
Most likely your speedo is wrong. As curtiscap said either tire size is off or the speedo is just wrong. Not uncommon at all. A lot of speedos read high. Keeps you a little safer with the cops and makes the car appear faster to the owner. Good for the younger guys who like to brag about how fast their car can go.
Look at the sticker inside the door jam of the the drivers door. It will tell you the recommended psi for your tires and the usually the standard tire size.
Also you can check the speedo using your watch. At 60mph you travel exactly one mile in one minute. Get out on the interstate and set your cruise control at 60. Then time how long it takes to go between mile markers.
If it takes longer than 60 seconds you're going slower than 60mph.
You can then set the cruise control at the speed where your GPS says 60 and time it between mile markers. Bet your GPS will be correct.
Look at the sticker inside the door jam of the the drivers door. It will tell you the recommended psi for your tires and the usually the standard tire size.
Also you can check the speedo using your watch. At 60mph you travel exactly one mile in one minute. Get out on the interstate and set your cruise control at 60. Then time how long it takes to go between mile markers.
If it takes longer than 60 seconds you're going slower than 60mph.
You can then set the cruise control at the speed where your GPS says 60 and time it between mile markers. Bet your GPS will be correct.
- Alohatoon1
- Posts: 48
- Joined: Mon Jul 13, 2009 2:01 pm
Re: GPS wrong?
Actually, satellites see hills and valleys pretty nicely. Even my $70 hand-lheld Etrex will give elevation readings that are accurate to within 10 feet.GXPWeasel wrote:To add a bit to the in a vehicle and hill situation, think about the speed for a minute. The GPS is triagulating from 3 dirrerent points to get your speed. These sattelites don't see hills or valleys, they just see how fast you are moving towards or away from them. So, it retrospect, your car may be going uphill, or downhill at 70mph, but you are actually going less than that to the GPS because you are going up/down a hill, and therefore aren't moving accross the plane ( earth ) as fast as your wheels are actually spinning. Much like the current and speedo situation.
I hope I didn't loose anyone there.
I don't think I'd bet my paycheck on it, but I think most GPS systems will work on an airplane in a vertical climb and still give good speed results.
Please visit my website if you get a chance, better yet submit a couple of your favorite photos there!
The Boating Photo of the Day
The Boating Photo of the Day
Re: GPS wrong?
Yes, tire size throws off the mechanical speedo, as the pickup for the speedo is either on a wheel hub or some part of the drive train calibrated to the factory spec tire size at proper inflation. Smaller tire makes more revolutions for given distance, drive train turns faster, speedo reads higher. Additionally, high heat conditions will throw mechanicals off as well, since such conditions make the tire bigger. Nitrogen-filled tires expand less than air-filled.
GPS units determine your position in 3D space, and given two or more positions calculate speed as 3D distance / time, so they are accurate regardless of elevation change. Time of day, obstructive terrain / vehicle surfaces, and weather can all effect GPS signal reception to some varying degree. If the unit can only get signal from two birds, it will use a series of plots to best-guess what it reports, perhaps by assuming the lowest calculated elevation, or most likely position from last good triangulated position based on speed and direction. Newer GPS units are generally more accurate, as they have a higher frequency of recalculation, and can make use of the extended information from the satellites that older units may not.
Those little water wheels are notoriously inaccurate in general as they rely on water flow and can't allow for widely varying turbulence and cavitation, and as stated, they are showing speed through water, not over ground, and certainly not in 3D space.
GPS units determine your position in 3D space, and given two or more positions calculate speed as 3D distance / time, so they are accurate regardless of elevation change. Time of day, obstructive terrain / vehicle surfaces, and weather can all effect GPS signal reception to some varying degree. If the unit can only get signal from two birds, it will use a series of plots to best-guess what it reports, perhaps by assuming the lowest calculated elevation, or most likely position from last good triangulated position based on speed and direction. Newer GPS units are generally more accurate, as they have a higher frequency of recalculation, and can make use of the extended information from the satellites that older units may not.
Those little water wheels are notoriously inaccurate in general as they rely on water flow and can't allow for widely varying turbulence and cavitation, and as stated, they are showing speed through water, not over ground, and certainly not in 3D space.
MrG
2006 32x10 Custom River Cruiser "Regardless"
2007 Evinrude E-Tec 250 2-Stroke
2003 Sea-Doo GTX Limited Supercharged
Ohio River - Evansville, IN
2006 32x10 Custom River Cruiser "Regardless"
2007 Evinrude E-Tec 250 2-Stroke
2003 Sea-Doo GTX Limited Supercharged
Ohio River - Evansville, IN