Wrong speed (speedometer vs. GPS)

 

Hi , I would like to know if anyone who has the nuvi 3790 lmt has noticed a difference in what their car speedometer shows and the nuvi ,mine shows a difference of 5 to 10 klms hr for instance if my car speed is 100 klm hr the nuvi shows I am doing 110 klm,I have a 2011 Kia rondo and asked the dealership from where I bought it from if the car speedometer could be wrong they said unlikely ,and 10 klms difference is not much ,but I think that might be the difference between a speeding ticket or not

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HPT
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Tire Size and Speedometers

Remember, the day your vehicle leaves the dealers lot, the tires lose small amounts of rubber in use. As the circumference of the tire becomes minutely smaller and smaller, the wheel has to make more revolutions to cover a set distance. (I'm retired GM, worked at the Tech Center in Warren, MI and other locations) For years speedos were mechanical devices driven by a gear off the output shaft of the transmission, through a cable up to the speedo. Since the early 80's depending on manufacturer and vehicle, they have been switching to electronic VSS (vehicle speed sensors) most are driven by a gear arrangement off the output shaft of the transmission but there are other arrangements as well, (another poster mentioned this). Until the time that a radar style system is installed on a vehicle that can 'look' at the road surface and measure true ground speed, you'd have these minute differences. Another system that relies on tire rotation to speed is ABS and Tire Pressure monitoring. ABS uses tone wheels or gapped inductor rings on each wheel coupled with a sensing inductor to sense individual wheel speed. Some cars use the 4 individual wheel speed sensors to detect a low tire. That's why some require a 'reset' or re-calibration at times, to zero the system. A tire low on pressure will turn at a different speed then the other 3 so a low pressure situation can be detected. Some vehicles actually have individual pressure sensors mounted inside the tire mounted to the rim and transmit via RF signal the actual pressure to the driver information system. Most get their power from a piezo device that generates enough current to broadcast the pressure value as the tire rotates.

There's lots of different ways to accomplish the same things on various vehicles.

A tip of the hat to you.

Quote:

Some cars use the 4 individual wheel speed sensors to detect a low tire.

I have wondered how the Toyota & the Scion could detect a small loss of air pressure. Now I know. One time the light came on while on a Sunday drive. The tires looked OK. I carry a tire gauge so I found the suspect & with my GPS I went straight to a Super WalMart where they quickly repaired the puncture.

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1490LMT 1450LMT 295w

You're Welcome

You're welcome Bob. A lot of vehicles just do a compare function on the 4 wheel speeds, the one that is low on pressure will turn a a different rate than the other 3, etc. The speed sensors are there anyway for the ABS system to detect unusual braking situations. On some vehicles they are used in the traction control system as well.....you can detect turns...left turn, the 2 passenger side wheels turn faster but their speed is equal to each other, than the 2 drivers side wheels, that are turning slower, but equal, etc.

Me too...

rthibodaux wrote:

I sometime look at my speedometer and the speed on my 3790LMT and it look accurate to me.

most times it's spot on! I still trust the car "speedo" that's the one the cops in Californis get you on, not the GPS's. Fun though!

--
"Backward, turn backward, oh time in your flight, make me a child again, just for tonight."

Not Unusual, Especially with Japanese or Korean Cars

I have a KIA and had a battle with my dealership about the accuracy. I ultimately timed myself between the kilometer markers on the highway, while on a flat section, with the cruise engaged.

This proved the KIA was consistently 6-7% high. Also passed one of those signs with the radar on it and it also confirmed the same error.

Honda had a class action suit against it, and had to add 5% to their warranties, due to odometer/speedometer being high. People were being denied warranty when the mileage went just over the warranty limit.

GM Speedometers seem to be dead on accurate.

What kind of car do you drive?

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NUVI2555LMT, NUVI350

GPS vs dash

bugdude wrote:

The 2 Nuvi's I have show going slower that the speedometer, by about 3 mph. but only at certain speeds!

Relative to GPS, my in-dash
odometer falls behind slightly. reads 198 miles or per 200 miles on GPS
speedometer reads slightly higher. 62mph or so and GPS showing 60mph

As you can see, I believe this is an intentional calibration error on the side of caution.

There was a class action lawsuit against Honda for odometer that read slightly ahead claiming premature expiration of warranty and accelerated vehicle depreciation.

Then, speedometer that reads slightly low can cause speeding ticket complaints.

Even if you were to have the speedometer adjusted (if it's even adjustable), it will still drift over time. As you know, tires get smaller as tread wears out. There is a manufacturing tolerance and variations within models and batch. If you check out tirerack.com, you'll see that tires with same designation will have different revolutions per mile.

Unless you're more than 5% off, the best you can do is calibrate the speedometer(calibrate, not adjust) every time you rotate your tires.
Memorize or attach a note in the dash at points that matter (i.e. 33=30mph, 65=60mph,77=70mph)

KIA Speedos Wrong

I complained that my KIA read about 6-7% high and the dealer wouldn't believe me. I tested it against the GPS and by timing between mileage markers on the highway. Also went past one of those speed signs that shows your speed.

Consistently, the GPS was right and the KIA was wrong.

Finally got the dealer top admit that others have had the same complaint and all had GPS'. However, he still wouldn't recalibrate.

Ultimately, I simply put next size larger tires on car, which was permitted and the speedo is almost correct now.

p.s. GM speedos seem to be almost dead accurate.

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NUVI2555LMT, NUVI350

Class action

GPSgeek wrote:

I complained that my KIA read about 6-7% high and the dealer wouldn't believe me. I tested it against the GPS and by timing between mileage markers on the highway. Also went past one of those speed signs that shows your speed.

Consistently, the GPS was right and the KIA was wrong.

Finally got the dealer top admit that others have had the same complaint and all had GPS'. However, he still wouldn't recalibrate.

Ultimately, I simply put next size larger tires on car, which was permitted and the speedo is almost correct now.

p.s. GM speedos seem to be almost dead accurate.

You need to take your complaint to the next level.

Look in the Owner's manual. There should be some contact info for Kia in it. When you contact them, explain what's wrong and how you determined it. Casually mention that there are a lot of Kias with this problem. Let them know that this could be grounds for a "Class Action" lawsuit (if they have them in Canada). Don't be threatening, but the mere mention of that will certainly grab their attention. Let them know that your warranty will expire before it's time and that you are not happy about being "cheated". When your odometer reads 50,000km - you will have actually only gone 47,000km (@ 6% error). And at 100,000km you will have actually gone only 93,000km. Don't know how many km the warranty is good for in Canada.

Kia may not even be aware of this problem and it could start a "Recall". I'm sure that it's only a setting that has to be changed in the programing somewhere.

The service rep at the dealer is probably totally ignorant of how to solve the problem. You might want to try another dealer (the bigger the better, as they may have seen this already).

P.S. My 2011 GMC Canyon is almost "Dead Nuts" at any speed.

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Metricman Nuvi 660, GTM-20 Traffic Receiver Nuvi 3597 GTM-60 Traffic Receiver Williamsburg, VA

I agree

I use my Garmin with several vehicles. Very rarely do I find any discrepensies between the GPS and speedo. I rely on the Garmin a lot for speed due to the fact some of the speedos are in MPH and i need KPH.

ghayman wrote:

The speed indicated on my 3790LMT and my speedometer are in accord. I drive with cruise control and I can easily see both.

When I tap my cruise control to increase speed by 1 mph, as the car's speed increases so does the indication on my nuvi.

I often use my cruise control to increase or decrease my speed and not use my accelerator.

I am very pleased how they are in agreement.

--
I drive, therefore I am happy. Rodeo, wildlife and nature photography rodeophoto.ca

But

t923347 wrote:

Here is what VW says about speedometer on my 2010 Jetta TDI Sportwagon:

"German cars allow the speedometer to read fast by a tenth of true speed plus 4 kph but never slow. The error may be higher or lower depending on wheel/tire sizes. "

For me the GPS speed indicator is the only one I look at.

But your warranty runs out earlier than it should

--
Using Android Based GPS.The above post and my sig reflects my own opinions, expressed for the purpose of informing or inspiring, not commanding. Naturally, you are free to reject or embrace whatever you read.

speed vs odo

There is a difference between the speedometer and the odometer. It is quite possible for the speedometer to read a bit high and the odometer to be spot on. So having the speed read high does not mean the warranty runs out sooner.

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-Quest, Nuvi 1390T

Speedo vs. GPS

The Speedo on my 2009 Maxima Sport Sedan indicates about 2 mph higher than what the nuvi 2460 LMT the GPS shows at 70 mph. The dealer insisted that speedo is accurate or at least within Nissan Spec.

After purchasing and installing Garmin's ecoRoute Mechanic the device reading is the same as the GPS satellite data.

NOTE: The ecoRoute Mechanic device is obtaining its readout from the car's on-board OBD II connector.

Conclusion: The Maxima's speedo is not accurate and the Nissan spec tolerance is too broad.

The speedo's on both of my Toyota's match the GPS readout.

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romanviking

True

ddeerrff wrote:

There is a difference between the speedometer and the odometer. It is quite possible for the speedometer to read a bit high and the odometer to be spot on. So having the speed read high does not mean the warranty runs out sooner.

This is very true!

To check the Odometer, you need to find a "Measured Mile", as you really can't rely on the Interstate Hwy mile markers. They are close, but no guarantee as to how close.

If you live in a city, this should not be a problem. Most cities require Taxi Cabs to get their "meter" checked periodically for accuracy. You can contact the branch of the local government that controls taxis for the location.

If you are out in the country (like me), contact the local office of the Highway Dept. They might be able to help. Or maybe a local or state police dept.

There used to be a "measured mile" right on the interstate about 4 miles from my house. For some unknown reason, it's now gone.

Modern spedometers/odometers are now digital. An onboard computer gets a digital pulses either from a sensor at a road wheel or from the drive shaft. The computer counts the pulses and every so many pulses it increments the odometer (the odometer on my truck is actually stored in memory somewhere and is displayed below the speedometer). The rate (speed) of the pulses is converted to control the Speedometer. My GMC Canyon has an analog speedometer display, but I dont know whether it is sent an analog or digital signal, but it's very accurate.

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Metricman Nuvi 660, GTM-20 Traffic Receiver Nuvi 3597 GTM-60 Traffic Receiver Williamsburg, VA

GPS Compared with Speedometer.

My 1995 Buick Estate Wagon and 1996 ImpalaSS both have electronically driven speedometers.

Unfortunately for comparison purposes they are both analogue. This gives rise to a parallax error between the needle and the dial. It's only a small error but exceeds the diference between the 855 and my intepretation of the indicated speedometer reading. I'm not willing to carefully lign up my eyes to eliminate the paralax error. Generally the error is only about 1 or 2 kms/hr or 1 mph. Too small to worry about.

Odometer readings are always slightly higher than the GPS distance. I assume that's because of the errors introduced when the GPS is tring to measure the distance around a bend.

For normal driving I prefer an analogue reading. However I also like the digital readout from the 855 when I'm setting the cruise control.

I never trust the maximum speed recorded by the GPS. Frequently it the number calculated when it is recovering from errors it made when it had incorrectly mrasured my position. e,g, assuming I was on the highway when I was actually on an exit ramp. When it sees I'm on the ramp, it alsmost instantaneously puts me on the ramp. Check your own trail records and see if that has happened to you.

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nuvi 855. Life is not fair. I don't care who told you it is.

iPhone vs Speedometer vs Nuvi

My 3490 and iPhone and Galaxy Note 2 show less than 1 mph difference. But the 2 vehicle speedometer one show 3 mph & 1.5 mph faster. So when the speedo show 70, the iphone or nuvi show 67.

GPS is much more likely to be right

Not2Bright wrote:

Odometer readings are always slightly higher than the GPS distance. I assume that's because of ...

My last car (a Ford) was off by several MPH on the speedometer at highway speeds and the odometer followed that (running the warranty out quicker than it should and adding milage to the car that further reduced its resale value). Don't just assume that the odometer is right and the GSP is wrong, it is much more likely to be the opposite. You can check this for yourself: on most major roads including all interstate highways, there are mile markers accurately placed every mile. Next time you are on a long drive, note both the odometer reading and the GPS reading at one of these, then note it as you pass another mile marker ten, twenty or more miles down the road (or, if you have a passenger, let them do this). Of course, avoid rest stops or fuel stops or anything else that might throw the reading off during the test. I expect that you will find the GPS is accurate and the odometer is off.

Similarly, if you maintain a constant driving speed on relatively level road, you can determine if the GPS or speedometer is more accurate. Check your speed between mile markers. Cruise control, driving at exactly 60 mph and a clock with a second hand all help, although it can be done at other constant speeds if you didn't get your math education in a public school.

...

I've found that a Ford Fusion *speedometer* reads slightly high, but this appears to be just an instrument reading.

If you put the car in debug mode and drive, the speed displayed in the message center is the actual speed ( about 2 mph lower than the needle and roughly matching the GPS speed). So I'm not convinced that the odometer is running fast at all (i.e. the computer *does* know the correct speed), but it would be good to check. As your tires wear, the wheels spin faster, making the odometer run (ever so slightly fast).

Mines...

My toyota shows about a 5 mph faster difference from my GPS.

.

Frovingslosh wrote:

although it can be done at other constant speeds if you didn't get your math education in a public school.

grin

--
Nüvi 255WT with nüMaps Lifetime North America born on 602117815 / Nüvi 3597LMTHD born on 805972514 / I love Friday’s except when I’m on holidays ~ canuk

Every Car

Every car I've ever owned has read several mph higher than the actual speed. It is rare, though, that a vehicle manufacturer will admit that it is built this way to reduce the possibility of inadvertent speeding. Still, I think that's exactly the case.

Fred

1450

my 1450 is also off about 5 mph, if I do not come at full stop a few seconds later then it read 0.

hard to believe

Driver 38 wrote:

my 1450 is also off about 5 mph, if I do not come at full stop a few seconds later then it read 0.

I find it hard to believe that a GPS (I'm assuming that is what 1450 reefers to) is off by 5 mph. It pretty much has to be much more accurate than that if it is going to be able to do its job. Or do you mean that it differs from your speedometer by 5 mph at highway speeds. I would have no problem believing that, but it would not be the GPS that was wrong.

Done that, frequently

Frovingslosh wrote:
Not2Bright wrote:

Odometer readings are always slightly higher than the GPS distance. I assume that's because of ...

My last car (a Ford) was off by several MPH on the speedometer at highway speeds and the odometer followed that (running the warranty out quicker than it should and adding milage to the car that further reduced its resale value). Don't just assume that the odometer is right and the GSP is wrong, it is much more likely to be the opposite. You can check this for yourself: on most major roads including all interstate highways, there are mile markers accurately placed every mile. Next time you are on a long drive, note both the odometer reading and the GPS reading at one of these, then note it as you pass another mile marker ten, twenty or more miles down the road (or, if you have a passenger, let them do this). Of course, avoid rest stops or fuel stops or anything else that might throw the reading off during the test. I expect that you will find the GPS is accurate and the odometer is off.

Similarly, if you maintain a constant driving speed on relatively level road, you can determine if the GPS or speedometer is more accurate. Check your speed between mile markers. Cruise control, driving at exactly 60 mph and a clock with a second hand all help, although it can be done at other constant speeds if you didn't get your math education in a public school.

Been there, done that and got the T-shirt.
On expressways, where bends are more gradual, over 100 kilometer stretches, the GPS and odometers agree to within 0.1 km. About 0.1% The markers could be out by that amount. Also my odometer only registers to 0.1 kms precision.

Over 2000 kms, including many off expressway routes, they disagree by about 20 kms, or 1.0%.

My conclusion stands unless someone can offer a better explanation.

--
nuvi 855. Life is not fair. I don't care who told you it is.

Speed

The vehicle manufactor's make them a little fast so they don't get taken to court because you got a ticket using their faulty equipment. They almost have to do it that way for that reason.

Speed difference

FZbar wrote:

Every car I've ever owned has read several mph higher than the actual speed. It is rare, though, that a vehicle manufacturer will admit that it is built this way to reduce the possibility of inadvertent speeding. Still, I think that's exactly the case.

Fred

I've learned that speedometer speed differences are often not linear - meaning maybe 3 MPH off at 10-40 MPH and 5 MPH off 50-70 MPH. Not always easy.

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Garmin Drive Smart 61 NA LMT-S

Speed difference

FZbar wrote:

Every car I've ever owned has read several mph higher than the actual speed. It is rare, though, that a vehicle manufacturer will admit that it is built this way to reduce the possibility of inadvertent speeding. Still, I think that's exactly the case.

Fred

I've learned that speedometer speed differences are often not linear - meaning maybe 3 MPH off at 10-40 MPH and 5 MPH off 50-70 MPH. Not always easy.

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Garmin Drive Smart 61 NA LMT-S

say again?

Garmin Gal wrote:
FZbar wrote:

Every car I've ever owned has read several mph higher than the actual speed. It is rare, though, that a vehicle manufacturer will admit that it is built this way to reduce the possibility of inadvertent speeding. Still, I think that's exactly the case.

Fred

I've learned that speedometer speed differences are often not linear - meaning maybe 3 MPH off at 10-40 MPH and 5 MPH off 50-70 MPH. Not always easy.

You can say that again! Oh, never mind!

Trusting my GPS

I have attempted to calibrate my speedometer in a variety of ways, and have found about a 15% error. When I finally got a GPS, it was interesting to see results very consistent with my previous findings.

GPS is accurate

I always trust a GPS over a mechanical speedometer. Most truck speedometers have a knob or screw on the back which you can turn to make it read anything you want it to read. The GPS is either accurate or not working.

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KD5XB in DM84
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