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GPS speedometer accuracy. Anyone ever question it?

shady

shady

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I've always figured gps is a pretty massive, expensive, complicated, and probably accurate system. That said I won't own a good speedometer for the simple fact of dead spots.
We had one in a work truck at my old job and it would just go to zero occasionally for a bit. Then come back when it felt like it.
I now use a phone app as one for now around town in my 4x toy, since it has no speedometer. I'm not too worried about it since I can usually gauge by other cars.

I've always "CHECKED" my speedometer in most vehicles by the GPS speed on my TomTom or now on my phone app. And a few times have made me raise an eyebrow :thinking:

I have had a few times in the same nearly new vehicle (few years ago, and a couple different cars) where I seen that the TomTom read a different speed than it had before vs my car. Like up to 6 mph different. And now with my phone app it has happened a few times in different vehicles too.

Example is in one of my rav4's I was seening how close it was on tomtom and it was always nearly identical. Then one day it's consistently about 4mph off. Another day it's back to normal. Then off again. So I started watching it on different trips from vehicle to vehicle. And it isn't always, but I have seen it a few times now. Again today I seen it in my CC. Speedo works. But I checked it when I got it and it was always on at 35 and off by 5 at 60. Truck read 5 slow... I've checked it a half dozen days because of the past and boredom.
Today it was dead on all the way to 70 :confused:

I'm just posting this short book:rolleyes: to see if anyone else has ever questioned their accuracy. ..?:dunno:

4 - 5 off occasionally is still pretty good though lol.
 
next time my Blazer decides to tear up a speedo gear im going to put one in it
 
All depends on satellite quantity and location. My phone app shows how many satellites it is receiving. 4 or less and accuracy goes down. Or if the satellites are on the horizon. It might have 6 in view, but if 2 or 3 are on the horizon, accuracy goes away.
 
Happened with the smart phone and app too
 
The GPS satellites themselves are part of a huge, incredibly precise system. Now that the deliberate inaccuracies have been turned off, they can be used to determine three dimensional location and speed quite precisely.
However, they do not watch all the receivers out there and send them individual location and speed data.
Instead they send out extremely accurate pulses that the receivers use to calculate their position and speed. Those calculations are fairly complex, and some receivers are faster and have more powerful processors.

Also, most receivers these days can track 12 or more satellites simultaneously. The older ones only had a couple of receivers inside and had to scan across the different satellites to get enough information to determine what it needs.
While a minimum of three satellites is necessary, the more the better, plus the greater the angle between the satellites the better the accuracy.
If the angles are poor, or if the receiver does not have a good lock on several satellites, many of them will interpolate what it has in order to deliver something instead of just a blank screen. If it loses all of them, such as a tunnel, most will say so, but with poor reception it just might wing it.

My M-Class uses a needle type speedo, and I hate it. In order to cram all the numbers onto the dial, the needle covers about 4 MPH. I stuck a GPS on the windshield several years ago, and very seldom even look at the car speedo any more.
My truck is fairly close even with the 12.50-33s, but I bought one of the "heads-up" display modules that lays on the dash and projects the speed onto the windshield several years ago, and it helps. If I were buying one today, I would get one that has the option of projection or direct viewing. sometimes the reflection is hard to see.
Also, if you consider one, double check to make sure its a GPS. They sell a lot of speedo modules that use the diagnostic plug to read the speed info from the car computer, and therefore are not any more accurate than the one in the dash.
If I were replacing the in-dash speedo anyway, I would certainly strongly consider a GPS one. But, otherwise, I would I would just buy a regular GPS and mount it on the windshield or dash.
Probably cheaper than the in-dash unit, and much more useful.
As for the blank spot problem, I think some of the in-dash units have an external antenna input that should help with that if so.
 
I've always used a stopwatch and mile markers as a third way to check accuracy. Granted the mile post markers may not be perfect, but it's a good gauge. Drive at 60 indicated, clock from post to post. If it is accurate you should cover the mile in 60 seconds.
 
I've never used the GPS apps for speed. To calibrate my speedo I use the mile markers and my odometer. My GMC DD has tires larger than stock, and using the above method I know my speedo reads 4% slow.
 
beags86 said:
Your problem is your tomtom, not GPS. Garmins don't have a problem...
Click to expand...

I very much agree. My Garmin, along with all of them that my coworkers have had, are all accurate and very consistent. That's actually the biggest reason I use it, the speed reading is often more accurate than whatever work truck I'm driving, and it's right up in my line of sight. That and the light blue tracer line that tracks where I've been. That's very helpful for my line of work too.
 
Have an app on my phone called 'gps status' fixes up to 24 satellites. I've never noticed it to be inaccurate when comparing to other sources. But you do need to have several connect.
 
I've been using Garmin Rino's for at least a decade now, and even though they are the better receivers (aren't nearly so sensitive to "shading" by foliage as the first generation non-HCx Rino's had) and you'll be hitting 6+ satellites, in an open field, the closest the receiver will show for accuracy is about 8-16ft when stationary. I don't know what that might translate to as an error when moving, usually movement helps, but even still, one would think sitting motionless with multiple satellites, error would be very near zero.
 
dyeager535 said:
I've been using Garmin Rino's for at least a decade now, and even though they are the better receivers (aren't nearly so sensitive to "shading" by foliage as the first generation non-HCx Rino's had) and you'll be hitting 6+ satellites, in an open field, the closest the receiver will show for accuracy is about 8-16ft when stationary. I don't know what that might translate to as an error when moving, usually movement helps, but even still, one would think sitting motionless with multiple satellites, error would be very near zero.
Click to expand...


This is a built in accuracy, or inaccuracy. Surveying equipment has been designed to an accuracy of .01'. The antennas are bigger and the processors have more calculating power.
 
ktmoutfront said:
This is a built in accuracy, or inaccuracy. Surveying equipment has been designed to an accuracy of .01'. The antennas are bigger and the processors have more calculating power.
Click to expand...

How does that compare to what is used within the car based GPS? I figured surveying stuff had to be much more precise, but I'm sure that comes with a cost penalty.
 
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dyeager535 said:
How does that compare to what is used within the car based GPS? I figured surveing stuff had to be much more precise, but I'm sure that comes with a cost penalty.
Click to expand...

Depends on the manufacturer. I would hope a Mercedes Benz would have a better system. But in reality, automotive GPS receivers only need to be accurate to 20 feet or so. Is the front or back of the car critical for location, or the center. Doesn't really matter on them.

And yes, surveying receivers and controllers are pricey.
 
I installed a speedhut gps speedo in my foxbody after i swapped in a viper 6spd, and there was no way to hook up factory gauges obviously. I had to have a small transponder sitting on the dash so the gps would work. Was very cool and loved not dealing with all the conversion wiring or anything. If i ever get another chance to build a square body, the speedo is one less thing to worry about when doing a modern drivetrain swap.

I compared it to various other cars and the aftermarket gps speedo was very accurate
 
I've been driving my '79 as my daily driver since the women in my family are getting in accidents. The speedo doesn't work in the K5, so I've been using an Android app. When I do drive my daily driver (non-K5) I check the app with the speedo in that car and it's within 2-3 mph.

I also check the app against the radar speed signs and the app is within 1 mph, so the app works.
 
I have a phone app that was right on for years. Been using it for at least 6 years . I checked with mileposts.

Unfortunately it's gone wonky. And now it sucks .

I've deleted and reinstalled to no avail.
 
I use HUD app for Android. Have calibrated a number of aftermarket speedometers in classic cars with it. Close enough.
 
I'm debating actually getting one for the 4x toy. Said I wouldn't, but enough people say they like them, and it's not my daily so :dunno: wth.
They're cheap enough and I only need it to be a general guideline on speed.
 
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