Speed accuracy?

Are the cars high or low vs the gps?

http://www.poi-factory.com/node/6695 =>Which do I trust? (speedometer or GPS)

Your speedometer in the car ALWAYS show faster speed. It is a rule for automakers to do it so. I have tested 3 cars already towards a laser speedometer. It is true. The automakers wants you not to speed up so high. Not all of them follow the pattern, but most of the US and German cars in US have this "quirk".

Just check your speed against these movable speedometers on the road, that show sometimes what your speed is when you approach speed limited section of the road.

The calculation couldn't be simpler: distance divided by time equal speed. Ex: 100 mi / 2 hours = 50 mi/hour.
GPS's are really good about measuring distance and time, so I'd place my money on the GPSr being more accurate than the mechanical speedometer in my car.

Noticed that on my car too. One of the mechanics at the dealership mentioned that car speedometers can be off as much as 5percent and still within specs!
Anyone else have experience with this?

I would trust my gps over my speedometer anyday of the week and twice on saturday and sunday. Even if the accuracy of your gps says 17-20ft, that will not make a difference in your speed reading because its only 20 feet. how far is 20 feet, what park 3 cars bumper to bumper and that is about 20 feet. that isnt that far of a distance think if it logically. i would go with the accuracy of your gps because they are constantly being measured for there precise location in orbit around the earth by the government and the government uses them for more than just navigation.

I have tried my nuvi 750 in four vehicles and found it to be within one or two MPH of the speedometer.

I have a c330 and my car is a corolla 1998 with 321,000 km, and the accuracy is perfect, 100 kmh on the speedo and the gps.

I have a nuvi 650 and I find it to be very accurate. The only thing is the 1 second refresh delay.

Interestingly enough it is never showing lower speed... always higher than you are going with.

A rule? I find that hard to believe. The speedometer in my Dodge Stratus (and the Intrepid I had before that) reads almost exactly the same as the speed indicator on my Nuvi 200, and my ETrex Venture CX. I see no more than about 1/2 mph difference at a steady speed of 65-70 mph, and that's about as close as I can read the car's speedometer anyway.

Many things can make your vehicles speedo off. Tire size is the #1 culprit. If they are different now than when brand new off the lot, the speedo will be wrong.

Recently I found out some other models like the 2007 Camry has accurate speedometer.

But yes... I have tested Ford Taurus, Jeep Grand Cherokee and Oldsmobil Allero from few years back.

The rest of the cars I can say that the speedometer shows higher speed, but I have not tested with a laser speedometer are some of the Chevies, Fords and Chryslers...
Some Toyotas and Some Hondas.

My Bimmer does it too. 4 mph more than the real speed.

tire size is the main thing that will change the speed on your speedometer. for example if you bought the vehicle with 30 thousand mile tires and then replace them with 60 thousand mile tires it will take more time for that tire to rotate around the axle, therefore causing your speedometer to read slower. and its the same in for the opposite if you put on smaller tires that what the vehicle came with it will cause your speedometer to read faster than you are actually going.

I have tested my Nuvi 250w in a 2007 4Runner, a 2004 Infiniti G35 Coupe and a 2004 GMC Yukon and the GPS is spot on with all of them as far as speed goes.

Guys, I work with sattelite navigation as well as ground navigation and I will trust my GPS device over any speedo known to man.

The technology behind GPS is a network of 24 orbiting sattelites using a technique called "triangulation" plus the precision of the U.S. Department of Defense.

Your speed on the GPS is calculated using an atomic clock, not some old tires on your vehicle.

Forget about the speedos on your car, when it comes to precise calculation of speed the GPS has only one competitor: Laser Radars.

Your Welcome,

-Sarcigon.

Only issue with the GPS accuracy is "lag". If you're moving at a steady, constant speed. It should be right on. Also, generally if your speedo is off, it's off by a percentage, not a certain number of MPH.

It is no secret that car speedometers can often be off by several mph. Your GPS is about as accurate as you can get for driving speed.

I've found that the speed indicated on my Nuvi 680 is dead on with my 2004 Impala. I know the roadside speed indicators in my area use regular radar since my radar detector picks them up. Regular radar is based on sound waves and uses the Doppler effect to calculate speed. The Doppler effect is an accurate measure of velocity however, the receiver can not distinguish between the many different reflections of sound bounced back at the receiver. This is one of the arguments of police using regular radar. My vote is for the GPS being more accurate.

just a fun fact that Ive known for a while, it takes 4 satellites to track your posistion. 3 to triangulate your position and the 4th one for time delays and multipath errors.

A car speedometer is based on the math of knowing the circumference of your tire, times the measured revolutions per minute. The accuracy of these measurements depend on the tire pressure, wear of the tire and how accurate the "per minute" clock in the car is. It would be impossible to argue that the car has an atomic clock inside it's speedometer, and it is highly doubtful if it has computational accuracy to determine the precise circumference of your tires at any given time and temperature. When I think about it, I'm amazed at how accurate our speedometers actually are.

GPS speed is based on an atomic clock in the satellite. But the receiver only samples this data every second or so. Then there is processing time to compute the distance between "where was I" and "where am I now" and then "how long did it take to travel". So with GPS, numerous samples would have to be taken to average out the processing and sampling.

For instantaneous speed, the speedometer will always be much more accurate because it is almost "real time". Measured over a longer period and distance, the GPS will be far more accurate.

Or worded another way.... The longer the period of time you are measuring your speed, the more inaccurate your speedometer becomes, and the more accurate your GPS becomes.

i have nuvi 750, it almost shows the same as my car shows. even though i beleve nuvi is more accurate.
-saravanan

There are different diameters between manufactures also.
Premium brands are full-size,
Some economy brands are smaller

Tire pressure can really change your Speedo also...a few pounds down can shorten your tires.

I tested this on my 2003 Taurus this last weekend and the C340 consistently showed exactly the same speed as my speedometer. So I must have the right size tires on.

Over the years I have noticed differences between the vehicle I was driving at the time and the GPS. They each may be off one direction or the other in different vehicles, but they are always off the same amount/direction in the same vehicle. Last year when complaining to the auto service department about what I thought was an inaccurate speedometer, they verified saying that a GPS was a better indicator of speed than the car's speedomter or even the local radar warning signs.

When driving my car's speedometer is about 3-4 Kph faster than what the GPS says.
2001 Subaru Forrester.

GPS makes a good replacement Speedometer

http://www.poi-factory.com/node/2410

While maintaining cruise control the GPS is on the nose. With normal driving, other than the usual satellite signal delay of ~1 second, the speed is always correct, same reading as my car.

I've tested the Garmin unit (same car) and the GPS is below the cruising speed ~3 Km/hr.

I've also tested the TOMTOM and the same applies with the Garmin results.

Hope this helps.

My Nuvi is dead on at cruise in my car. It is off on my motorcycle, but motorcycle speedos are notoriously off.

My speed is also right on with all of my cars. Dodge Durango, Chrysler Crossfire and Dodge Viper (when I have the stock tire sizes on). Note newer cars with ABS normally use the average of all 4 tires in determining the speed based on wheel speed sensors. Older cars or those without ABS may be less accurate as the speed sensing is normally done in the transmission by a pinion gear. Pinion gear steps preclude super accuracy given that most cars have multiple tires sizes available and the pinion size chosen is the best for the range of tire sizes.

Plus what was said above about tire wear, inflation and replacement tire sizes (note the same size tire can have measurably different rolling radii for different brands of tire).

I find that my 660 and my spedo read precisely the same speed, however, a GPSr will always be the most accurate due to triangulation by multiple sats. As others have posted, the tire size, inflation ect. variables all weigh on your spedo accuracy.

You can trust the GPS. I have altered the size (diameter) of the tires on my car and have gone to the manufacturer's site as to "speed relational differences" because of the size (14" to 16" plus the size of the tires themselves) to get different diameters from the "factory" settings. The GPS is dead on!

I love the digital age!

My c530 is usually 1 mph lower than my Jeep Grand Cherokee. In my Mustang, the c530 is 4-5 mph lower. These findings are at average speeds between 20 and 70mph.

If you doing 80mph or more, it is safe to say that most cars begin to lose accuracy in their speedo.

I'd trust the GPS more or at least know the comparison to your own car, so you know roughly how fast your going.

But why 80mph? These days, they just count pulses, convert it to an analogue signal and feed it to a voltmeter. On a VW/Audi speedo at my disposal, they add 10% upto 40mph and then +5mph after that.

They don't do this in the Trip Computer though - that uses the real values. They do use the incorrect figure for the 'overspeed' warning, that you can set.

The only car I tried where the speedo was spot-on, was a Mercedes C180.

Quick test: Put car on cruise control at 50mph indicated. Reset Trip Computer and observe 'average' speed (which of course is your current speed). Whenever I've tried this, it's always agreed with the GPSr and not the speedo.

Of course, strictly speaking, you can only quote the speedo's reading to the nearest 5mph (or whatever the smallest division is).

Thanks for all the replies - definitely an informative consensus, and it validated my gut feeling.

Interestingly enough (though I could be wrong), I think tire circumference has long been eliminated as a source of speedometer calibration issues. At least in my past two vehicles, the oldest being a 96 Ford, they didn't use wheel/axle rotation to measure speed. It was done with another velocity measuring device. I only know this because I had to have it replaced once. I will have to look it up to satisfy my curiosity or to see if I am completely off base.

Partially answering my own question, from the Haynes repair manual:

1996 and later models do not use a conventional speedometer cable, they use an electronic speedometer and a Vehicle Speed Sensor (VSS).

How it works... No clue. Maybe it's still tied to the wheels somehow.

GPS wins the speed question.
I've installed two GPSr's in my truck (sp 2720 & sp 7200). They both measured the EXACT same speed to the tenths. My speedometer in the truck was off by 2 mph.

Just my 2 cents worth. (To bad the 2 cents cost over $1500!)

Wrong. All cars use the tire rolling radias. Some through a direct measurement of drive wheel rotation (usally through a output shaft speed sensor in the transmission) or through the use of wheel speed sensors for the ABS system. But as the tires are the only thing in direct contact with the ground they must be involved in the speed measurement. No car made currently uses GPS, radar, laser or other non-contact speed measurement method.

Most modern motorcycles do, though. They have electronic speed sensors that are not connected to the wheels.

I'm pretty sure that some cars do, as well.

As a former State Trooper who still owns a RADAR unit, I would trust the GPS over any speedometer. I have run several tests on my two cars and a friend's and the GPS and RADAR agree 99.9% of the time.

shows about 3 MPH under the speedometer.

My Nuvi 660 and Acura MDX seem to be in synch regarding speed.

I guess your cars give a new meaning to the word "Sub-Compact".

when i setup 35 MPH for speed camera, when i drive on FWY, gps will be show me over 35 MPH, why ????

How did you set the 35 mph speed? Are alert turned on on your GPS unit?

And you need to understand that setting a speed alert only functions to alert you that you have exceeded the speed set. It will not prevent you from traveling over that speed.

I have a Toyota Avalon and while driving on the freeway near Tucson where the road is flat and there are no overhead obstructions, mountains, or anything else to interfere with the signals, my speedometer and Nuvi 680 both showed the same speed. I tried it at 65, 70 and 75 mph and both indicated the same speed.

From what I have learned from various forums, the Honda Gold Wing is well know to have the spedo show a higher speed than the bike is actually going. Good thing is it helps keep you from speeding, bad thing is that it racks up miles on the odometer that you did not drive.

Well I would be very interested to find the technology that a motorcycle uses for speed measurement that does not use wheel rotation. Call me a doubting Thomas but as an engineer at a major automobile OEM I find the claim unrealistic.

To add to the survey ..I have a C530 and it shows the same speed as our Expedition.

Okay, Doubting Thomas.

As you can see in this page from my Service Manual, the vehicle speed sensor plugs into the side of the engine. There is no connection to either wheel.

http://i26.tinypic.com/epqcz7.jpg