What to believe (ScanGauge & GPS vs. speedometer)
 

My GPS and my SG II agree on my speed. The speedometer shows about 3-4 mph lower than both. I tend to think the GPS and SG are more likely to be correct. What does the group think??

I believe the SG and the speedo are getting the same info from the computer. I think maybe the actual speedo needle isn't very calibrated. The trip odometer is accurate to within 1/10 mile for every 100 miles driven.

Thoughts?

I would go with the GPS and SG.

H4MM3R -

I agree. Hmmmm. How would you check your GPS accuracy? Maybe walk around a high school running track 4 times and hope it adds up to one mile?!?!?!?

CarloSW2

I used mile markers to check my GPS. Get up to a GPS reading of 60 mph and time yourself between markers. If it takes one minute from marker to marker, it's more or less accurate.

Funny how the OBDII port tends to know the real speed and distance - meanwhile we lose 4% of our warranties! :confused:

My GPS shows that I'm 1-1.5 miles per hour off on my speedo. Other people driving behind me tell me that I'm doing 50 when I'm doing 55, but it is THEIR speedo's that are way off. I just can't convince them that, because their cars are newer... pshhh, oh well.

Who -

Yeah, Honda got a black eye for that :

Honda paying out $6 million for 'overclocked' odometers - Feb 19th 2007
http://www.engadget.com/2007/02/19/h...ked-odometers/
CarloSW2

Oh.. that sucks.

Why don't they calibrate speedo's at the factory? I think that's the reason cops don't pull people over anymore unless they're doing ungodly fast speeds because of speedometer error, they could be thinking their doing fine with their speed, but they could have a speedo that's 10mph off like my grandparents RV.

DPOV - Speedometers and more importantly odometers never read slow with any factory tire sizes, and the error increases marginally as the tread disappears. In fact by the time you are speeding it should be obvious in many people's speedometers since the indicated speed will be higher than the actual. I can't see cops giving any benefit of the doubt on that.

some people's speedo's read low, yet they are going faster than it says.

Well, I guess it's just my tires, cause they're almost to the replacing point!

What type of GPS is it? if it is a portable one, they tend not to be all that accurate, if it is an in-dash GPS with the speed sense hooked up, then they are relatively accurate.

Why would a portable GPS automatically be less accurate? The math involved in GPS triangulation isn't all that complicated and in all likelihood, the GPS modules in in-dash variants (technically, mobile :p) are similar if not exactly the same.

That being said... And answering the OP. I had three GPS's running in my car at one point. One was over cell, one was for navigation used for geocaching (didn't have road maps etc.) and the third was a Magellan 3xxx. Everything was dead to nuts equal and matched with the SG :) My speedometer, on the other hand, is 5mph slow at 70mph :eek:

The portable GPS units are less accurate because they don't use the speed sense signal from the vehicle in their calculations like the in-dash ones do. Combining the actual speed sense signal with the GPS triangulation, makes them more accurate.

Well... Mine gets it's speed from 12 different satellites at one time... It's portable. I believe it, because it's only showing 54 when I'm doing 55 (speedo). And apparently my bud behind me in his geo said I was going a little slower than 55.

reformed -

To me this is counter-intuitive because the point of the GPS is to get a reading that is independent of the car's on board sensors, which may be inaccurate.

I do have one caveat. When I was driving with the GPS, I left it front and center on the dashboard, wedged up against the windshield, to insure good signal strength. If I left it in a side pocket or on the floor, it wasn't accurate. My logic was that I didn't want it to lose the signal before I made a right turn, and then just have it "connect the dots (aka the hypotenuse distance of a right triangle)" when it got the signal back.

CarloSW2

I don't buy it... I'd hope in dash units with a speed sensor line use it to calibrate to the GPS network (or when GPS signal isn't available). In any case, that doesn't answer the question of why. Why would a GPS be less accurate? I mean, our airlines rely on the same exact satellites - the military relies on the same exact technology... Even large ships rely on it. And they don't have a direct reference to a stationary "ground."

GPS signals propagate at the speed of light - much faster than signals from a hall sensor.

Hey, I'm just going by personal experience. I install these things for a living, along with other car electronics and accessories, and I've noticed a trend over the years. I've actually had portable GPS and in-dash GPS units in my car multiple times and from what I've noticed, the in-dash units seem to be more accurate on the whole. I'm not claiming I know exactly why, just using reasoning to know that the speed sense is really the only difference between the two, and making my assumption.

GPS signals aren't 100% accurate on these portable navs.. you aren't paying for the technology. If you've had them in your car, you can see when you are driving and you pass a street, and then look at your nav and you will see the street either before you or after you on the screen. Sometimes its pretty close, sometimes its way off. When you are using the turn by turn features, sometimes when you come up to an intersection and you turn, the nav shows you turning too soon or too late sometimes when, in fact, you are turning into the intersection perfectly. This doesn't happen with the in-dash units that I've used, at all. Maybe because they cost more, they incorporate better technology, or maybe its the speed sense, I don't know that for sure. Believe me or don't, all I am saying is that I've installed 100's of these units and this is what I've noticed.

Pinpointing location is far different than calculating speed on the GPS network ;) IIRC the current standard for civilian GPS is 15 feet (compared to P(y) encrypted millitary which is somewhere in the tens of centimeters) - but if you're using WAAS, that drops down to 1-3 meters thanks to the GPS ground stations :) But that still doesn't change velocity accuracy from relative measurements. It does play a role in location accuracy - but this accuracy goes up as time at that location increases.

I too am just passing along my experience :)

Out of curiosity - what models do you install?

Isn't the difference of whether your on the street or beside it is because (if it's true) the tectonic plates move about an inch every year? and satellites have to be calibrated every so often?

The satellites are calibrated more than often :) Quite frequently. If they weren't, their internal clocks (which need to be sync'd) would get quite off rather quickly (the whole faster your travel the slower time moves thinger :p)

But even with moving plates and such - that doesn't change your position (we don't update the latitude/longitude maps based on continental position). And that said, you don't need to know exactly where you are to know how fast you're going - you just need a relative current and previous position (and time). Think of "Back to the Future" - "We don't need Roads" :D

GPS hardware designed for road navigation will probably fudge map displayed position a little if you're close to a road to show that you're on it. I've noticed that in more rural areas - but that doesn't mean the GPS is off (could - heaven forbid - be the map itself is slightly off :p)

As far as the portables, it varies, usually Delphi, Harmon Kardon, or Garmin lately. Sometimes Pioneers version the avic S1. The in-dash units are mostly Pioneer, some Eclipse, some Alpine (years ago) and a few Panasonic. I personally switched from the Delphi, to the Pioneer in-dash, and its a world of difference.

I also forgot to mention that the in-dash units have a built in gyro sensor to compliment the GPS, which makes them more accurate as far as location goes. You seem to know more about the actual workings of GPS than I do, I just put them in, take them for a test drive to make sure everything functions within tolerance, and send them on their way. So I will say I believe you that speed and location are different.