Pontoon Boat & Deck Boat Forum

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

My GPS is dead on accurate on the boat.

It your truck make sure the GPS isn't obstructed by the cab it will shield the signal and cause inaccuracy.

thanks guys il have to put the gps up on the dash and see if it stays steady

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.

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.

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.

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

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.

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.

LOL that makes sence and with me living in the mountains its ALL up or down

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

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.

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.

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.

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.

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.

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.