Which do I trust? (speedometer or GPS)

 

My nuvi 650 tells me I am going one speed, my spedometer tells me I'm going a few MPH slower. At 70, by the nuvi, it says almost 74 on the speedometer.... I'm inclined to think the nuvi has it right..

--
John - with a Garmin 650 and a 750
3 4 5 6 7
<<Page 2>>

Here is a

Speed

Again, unless the auto is a police vehicle with a certified speedometer for clocking speeders, I would not rely on a stock speedometer. The GPS is most likely exact. Jake

Every car with an analog

Every car with an analog speedometer that I have checked while driving is almost 100% in sync with the GPS up to about 70mph. After that the speedometer rises a lot faster than the speed on the GPS.

In my experience, digital speedometers are more accurate than their analog counterparts, however there are significantly fewer of them around to fully test.

Overall though, aside from a radar gun, you can't get much more accurate than a GPSr.

tire size

amontman wrote:

When I went to a larger tire size my repair shop connected his computer and set my cars computer so the tire size would be calibrated with my speedometer. Tire wear will change the accuracy so I always trust my GPS.

Ah, the joys of total electronic systems. You can tweak them. My truck uses the old cable down to the transmission.

--
Rusty Myers Austin, TX Garmin eTrex Vista, Zumo 550

Not all speedometer are

Not all speedometer are built the same. In some car magazine, under specs, when some vehicles are put on dyno machine (dyno meter) it shows that the speedometer and the reading from dynometer aren't equal, rarely the speedometer is lower than the dynometer reading.

So the GPS reading is equivalent to dynometer, so naturally trust your GPS.

--
--------- Bloody Garmin, those features in Nuvi 7xx should have been incorporated in Nuvi 6xx!!!

GPS

I would trust the GPS all the way. Vehicle speedometers are factory calibrated to be off.

GPS absolutely....

GPS absolutely....

--
NickJr Nuvi 3597LMT

yes, this is true and I have

yes, this is true and I have checked it always. helps a lot.

yes, GPS is much much better

yes, GPS is much much better than the speedometer.

that radars cannot be used

that radars cannot be used in different states.

Speedometer reading

It's been a few years since I've been involved in vehicles where I knew a bit about how things were designed to work. But the last time I was, the point where the speedometer takes it's reading is on the transmission output shaft, not the wheels themselves. That doesn't change any of the conversation regarding wheel sizes, inflation rates, tire wear & tear, etc affecting speedometer readings since there is a direct relationship between wheel speed and output shaft speed, but I just thought I'd pick that nit.

--
--- GPSmap 60CS, Nuvi 650 & Nuvi 1490T---

BINGO!

asianfire wrote:

it really boils down to, What does the policeman's radar unit say that you were going......lol.

Too true! smile

Regards, Ted

--
"You can't get there from here"

That has to do with Occasional

badliquid wrote:

I think the GPS is less reliable, and my reasoning is because when I look at what it says my max speed is, it says 143KM/H, which I haven't been up to that speed in at least 10 years, I don't go past 115KM/H.
I'm not sure why its saying that, but i ignore it, i know i'm not going that fast.

glitches in reading the satellites or some such thing...my wife's C330 says her max speed is 3728 mph...must have been when I was driving smile

Regards, Ted

--
"You can't get there from here"

Radar or Radar detectors?

arun_ng2000 wrote:

that radars cannot be used in different states.

Huh? Radar can be use in every state! So as long as you have a license to operate it. There are few guns that you can operate (which falls under fcc part 15) legally without a license. The rest fall under part 90.

Now, if you are actually talking about radar detectors, they can also be used in every state except for VA, the District of Columbia, and military bases.

--
Charley - Nuvi 350 - Bel STI Driver - Cobra 29 w/ wilson 1000 - AIM: asianfire -

digital speedometers

akirby37 wrote:

snip...In my experience, digital speedometers are more accurate than their analog counterparts, however there are significantly fewer of them around to fully test....snip.

Digital speedometers still rely on an analog measurement of some part of the drive train, and therefore only indicate how fast the vehicle's mechanical parts are rotating. This may be a highly accurate measurement, but it's not measuring the interface between the vehicle and the surface of the earth. If you spun your tires, your speedometer would show you moving at a high rate of speed. Slam on your brakes and your wheels stop, as would your speedometer, but your car would continue to move.

GPS units are 100% digital and measure speed over the ground, independent of the mechanics of the vehicle.

--

New Rims?

If you changed your rims and tires that would change the accuracy of your speedometer.

Yes, almost certainly.

sfgps wrote:

If you changed your rims and tires that would change the accuracy of your speedometer.

Unless the geometry happened to stay the same (and that would be a coincidence if you changed both the wheels and tires), yes you would. There is some pretty detailed discussion of how that works earlier in this thread if you'd like to know more.

--
--- GPSmap 60CS, Nuvi 650 & Nuvi 1490T---

Honda do not trafic speedometer

Sorry to tell you that TOPRUDDER but it's false , Honda do not do this or other company.

The reason of that error is the marge of error of the equipement , sometimes 3%. Everything comes into the specifications. GPS will always be more accurate due to the clock satelite. The difference from your Honda or other cars is due to the wheels size who can not be exactly the size you think. Tires has also somes marge of error. Just add the multiples errors and you will get the result. Calibration error of the speedometer (analog or numerical)is always present.Numerical one could be a little more realistic sometimes and look more stable in reading.

Thats why a speedometer cost almost nothing for company. Ground Speed measure is always more accurate with GPS.

I have notice on my saturn

I have notice on my saturn the speedo is a little off with my GPS. In my Jeep the speedometer in right on with the gps.

--
Paul Team Dougherty

does not matter

It really does not matter which one is correct the nuvi or speedometet. All that matters is what the police radar gun says at the time of the incident. Unless you can take a picture of bot the nuvi or speedometer to prove him wrong....lol

Your right

Your right.

Who care , it's just a couple of miles per hour less or more. Anyway i never look my speedometer.
This case should be close now.

Lets go to another subject.

Car

I would go with speedometer

--
[URL=http://www.speedtest.net][IMG]http://www.speedtest.net/result/693683800.png[/IMG][/URL]

GPS is definately right/accurate

IIRC, the allowable tolerance for speedometers in Europe is +(10%+5km/h) / -0

And Honda US settled the

And Honda US settled the resulting Class Action. Reason? +3 KPH = a LOT of extra miles on the odometer making the warranty expire sooner than it should and reducing the value of the vehicle on resale.

Honda Canada has not settled yet.

come on now, everyone has

come on now, everyone has known for yrs and car speedometers r not rt. in fact i believe mfg r required to make them accurate within 2 mi per hr, and any way the god of speedometers(gps) has got to be rt. i never thought of the poi factory being a church. oh well its sunday.lol

Trust your GPS

Trust your GPS. Car manufactures face fine$ if their cars read under; this is because it would provide speeders a way out of speeding tickets. VW has issued a TSB for my 06 because the speedo reads fast. After the TSB has been performed the % error is reduced but it does NOT remove it.

Speed

I work for a Infiti dealer, and on the in board GPS it reads 2 to 3 kmh diff with the speedo and the company nows about it. since it is over than the GPS so no chance for speeding ticket.

--
Claude using Garmin c330,Nuvi 250W and a Etrex venture Cx. Member #2602

Speedometer or GPS

I am more inclined to believe the GPS over the speedometer. At lower speeds, both seem to agree, but at highway speeds (55 and above) there is about a 2-3 mph difference, with my speedometer the higher of the two.

If you think auto

If you think auto speedometers read fast you should check out high performance boats. They typically read 6 - 8 mph fast. In performance boat circles everyons says that the biggest drag on a boat is a GPS as you always lose 6 - 8 mph top end.

--
Padraig

i would go with the

i would go with the speedometer

GPS Respond time?

Does GPS respond time can affect the speed reading? I think most new models have high respond time to calculate the location. But I am not sure the slow GPS respond time can give a lower speed reading..

--
Newport

My Results

I've compared the speed of my GPS to my three vehicles, which are a 1993 Yamaha Virago, a 1996 Nissan pickup, and a 1998 BMW 528i. The speedometers of my motorcycle and my truck show exactly the same speed that my GPS shows. The BMW's speedometer shows that I'm driving five MPH faster than what the GPS shows.

I still don't know which of the speeds are correct, but I was surprised that my motorcycle's speedometer showed the same speed of my GPS.

--
nuvi 285WT

VW/Audi adjustments

TheBug wrote:

VW has issued a TSB for my 06 because the speedo reads fast. After the TSB has been performed the % error is reduced but it does NOT remove it.

Are you aware of this software (and hardware)?
arrow http://www.ross-tech.com/vag-com/download/

Using this, you can customise all sorts of things on a VW/Audi(/Skoda/Seat/Masserati/Lamborghini).

I've had great fun with it wink

--
------------------------ Phil Hornby, Stockport, England ----------------------               http://GeePeeEx.com - Garmin POI Creation made easy           »      

GPS Wins

I have a calibrated speedometer. That does not mean that my speedometer always shows the correct speed, it means that I have a document that reflects the error at a given speed.

For those of you who think the GPS is wrong because of some delay, remember that the delay is consistent. If my reading was .05 seconds slow a mile ago, it is .05 seconds slow at my current location. The math is still good. If you stomp on the gas to increase speed, you new speed rating will also be .05 seconds delayed.

I just got radar installed in my car, so I will report any inconsistencies when I see them. (I couldn't find my Garmin when I got the radar, as I got a new car to go with the radar. Everything is sorted out now.)

My 260....

...is dead on with my Honda Accord, but was just 1 or 2 mph off on my father's Buick at 70mph. I checked it several times on a 2200 mile trip to Florida, and it was consistent... speedometer at 72, GPSr at 70. That was on the freeway with cruise control on.... I believe the GPSr.

--
Rick - Nüvi 260 - eTrex Summit HC

GPS VS RADAR

As I am driving around with the Radar on, and GPS active as well, the two devices are usually dead on when I am at a steady speed. During acceleration (or braking) the GPS lags behind the RADAR for about a second. (generally no more than 2 mph.) Around 40 miles an hour, the GPS will be off by 1 mph with the RADAR, but I don't use cruise control, so it could be a rounding issue, as they do sync after a moment. I just figured out that I can have a digital display of my speed on the dash (new patrol car), so maybe I will try to see how all three devices sync up.

GPS is 3D not 2D

Ein wrote:

Which would you trust if your are going up or down a mountain?

Still, I would trust the GPS. Your map may be 2D, but the GPS calculation is 3D. Many GPS's will tell you altitude as well.

So it doesn't matter if you are travelling perfectly horizontal, up or down ... your position is calculated at two 3d points and the speed is calculated on how fast you got between those two points .. left, right, straight, up or down.

Speeds

Speedometer...Period!!!

Speedo VS GPS

Take the average and for-get-about-it! Too many variables!

Not even a close contest

GPS....

--
John Nuvi 750 765T Winnipeg, MB

Sometimes

If you have the original tire size at correct inflation,and they are not worn, then it is possible for the speedometer to be very close.
At steady speed, a good GPS will be accurate.

The Best reason to trust the GPS

The best reason to trust your GPS over your speedometer is because the speedometer has only one source of reference for how fast you are going and so far as I know that is the speed sensor in the transmission. The GPS with proper signal strength is pinpointing your exact location of off as many satellites as it can find and constantly recalculating your position and how long it took you to get from your last reported position to your current position. With as many as twelve satellites all agreeing that you are where your GPS says you are I would stake my entire months salary on the speed being reported by my GPS. As for the outrageous max speed being logged that is as simple as one time you lost signal or connecting to another satellite that reports your location slightly different for just a split second.
I hope that this information helps.

My truck

In my work truck I would have to say the GPS is more reliable. The speedo has been out of commission for about 2 years now. I use the "I think I am going this fast" method.

--
----- Magellan Maestro 5310 ----- Free Garmin Nüvi 270 -----

Honda Owner

I own a Honda with the speedometer problem. Gotta look it up in my records but I seem to remember my warranty was extended due to the Honda's speedometer being off. From what I can tell mine seems to be off by the 3 mph.

.

bumpusd wrote:

The GPS with proper signal strength is pinpointing your exact location of off as many satellites as it can find and constantly recalculating your position and how long it took you to get from your last reported position to your current position.

Actually, GPS units calculate speed by the doppler shift of the satellite frequencies. They don't derive it from the positional data that the GPS is calculating. This allows the speed calculations to be extremely accurate.

Because they don't rely on positional data the speed is equally as accurate whether your positional error is 3 feet or 60 feet.

As long as you've got at least 4 satellites, your accuracy will be less than 1/2 MPH.. In their specifications, Garmin claims the NUVI units are accurate to 1/10 MPH with or without WAAS.

i would go with a GPS

i would go with a GPS

Funny

When I first got my GPS, I was showing my wife the features, and she was driving, and I showed her that the GPS was reading her speed as 62 MPH, and she looked down at the speedometer, and remarked, "But this says I'm going 60". I told her to give the GPS a break, it was afterall telling her how fast she was going by using some satellites orbiting the Earth, and 2 MPH difference was pretty good. I told a friend that later, and he remarked that the GPS is more accurate because the condition of the tires will change the speedometer reading.

Speedometer accurate if....

If you are in a late model car with original tires, wheels, etc, the speedometer is generally more accurate. If you have a digital readout on your speedometer, that would eliminate any parallax error reading the speed as well as needle width and presentation errors that may reduce the accuracy of the speed you read off the meter.

The GPS can be subject to a number of factors that may make the speed less than accurate. However, if your accuracy of signal is good (15 or less) and your speed is constant, the GPS speed should be completely accurate. I say your speed should be constant because each GPS has a different update rate and if you are changing speeds faster than the update rate, or of there are signal anomalies, your speed will be inaccurate.

Bottom line to answer your question: Depends!

This was in court

Funny how this topic came up. About a year or so ago a kid was pulled over for speeding. His dad had a gps tracker service installed in the car as his son was a teen and he wanted to monitor his speed and locations. Well, they pulled up the info on the web and it showed he was not speeding. They went to court with the GPS company and fought the radar gun company. The winner?... The GPS. It was proven that the GPS is more accurate then a radar gun. I would never trust a car. Everything from tire size, pressure in the tires, wind..ect can throw the speedo off. It became a big deal because of the fact that more people own a gps device and the radar/law people are now worried that more people can and will fight it.

I wonder how big the ticket was

slowjazz wrote:

Funny how this topic came up. About a year or so ago a kid was pulled over for speeding. His dad had a gps tracker service installed in the car as his son was a teen and he wanted to monitor his speed and locations. Well, they pulled up the info on the web and it showed he was not speeding. They went to court with the GPS company and fought the radar gun company. The winner?... The GPS. It was proven that the GPS is more accurate then a radar gun. I would never trust a car. Everything from tire size, pressure in the tires, wind..ect can throw the speedo off. It became a big deal because of the fact that more people own a gps device and the radar/law people are now worried that more people can and will fight it.

I wonder how big the ticket was to go to all the expense of fighting it in court. Additionally, why go to the expense of adding a GPS Tracker unless the kids track record for speeding was costing more in fines than it was costing to have it installed.

--
Jihad THIS!! Political Correctness is a doctrine fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
3 4 5 6 7
<<Page 2>>