Car speedo and GPS

[packerjim]

I had my new Garmin Etrex H in the car with me yesterday and I noticed the speed reading on the GPS was approximately 2 miles slower than my cars` speedometer . Question is the GPS more accurate or the speedometer? I do notice though on those speed reading gizmos the police here in town put out off the road that my speedometer is pretty much a match .

[+Gitchee-Gummee]

Too many variables to determine which "one" is the most accurate.

Vehicular speedometers are merely approximate speed indicators and are notoriously poor indicators at best. Your speedometer may be dead-on at one speed, but off by as much as 7 - 8 mph at another speed (usually a higher speed is less accurate -- not always). This is why police depts (as a rule) allow a 5 mph "grace" in their departmental policies.

 

The speed indicator or your GPSr "could" be considered more accurate, but it too, has variables.

 

Take your pick.

[+Cowboy Camper]

I can vouch for the GPSr having it's own variables to it. I had two GPSr's in the truck yesterday and they both read a different speed. Only 1 or 2 MPH or less difference, but different none the less.

 

Still, I would trust the GPSr over the car or truck speedometer. Even Tire Wear can change the speedometer in a vehicle to an extent. As for the police speed reading gizmos, those work like radar and can also be 'fixed' if ya know what I mean.

[GermanSailor]

Question is the GPS more accurate or the speedometer?

The GPS is more accurate. The speedometer gets its information from the gearbox, but isn't usually compensated for slight differences in the circumference of the (driven) wheels. On the other hand, car speedometers have to overread in order to prevent you from speeding. At least this is the law in Germany.

 

GermanSailor

[+GrateBear]

I agree with the GPSr being more accurate. The first time I used one to check, the speedo read 60 and the GPSr was 54. So, I went to a forum for for my car (BMW) to ask what others could tell me about the speedo, and learned that BMWs have a 7% margin built in (on the high side, obviously). Interesting to see the post from GermanSailor about it being the law in Germany.

[+Bundyrumandcoke]

I always run both my GPSs when driving my 4wd, or riding the bike, even if not caching. One or both is always on speedo mode, as the speedos on both vehicles are inaccurate. I would much rather trust the GPSs.

[+DanOCan]

Speedometers been calibrated to read high is one of those sneaky little car things, much the same way that when the gas gauge shows 1/2 tank you actually have used more than 50% of your capacity.

 

My experience is exactly like yours -- with the speedometer showing 100km/h my GPSr consistently reads 97km/h.

[knowschad]

Question is the GPS more accurate or the speedometer?

The GPS is more accurate. The speedometer gets its information from the gearbox, but isn't usually compensated for slight differences in the circumference of the (driven) wheels. On the other hand, car speedometers have to overread in order to prevent you from speeding. At least this is the law in Germany.

GermanSailor

That sounds about as bright as setting your alarm clock ahead so you don't oversleep! I used to do that, until I found myself sleeping 10 minutes late because I knew my alarm was set 10 minutes ahead.

[+Prime Suspect]

A GPS determines speed by detecting the Doppler shift in the satellites' D-band signal(s). A consumer grade GPS can be as accurate as 1/10 MPH under ideal conditions. Under normal conditions, it's more like 1/2 MPH accurate.

 

Sampling is done in 1-second segments, so if you have 2 GPS units in the car at the same time, they may very well show different speeds because the segment samplings won't be in sync. It's not exactly like comparing apples and oranges. It's more like comparing apples to different apples.

[+Cowboy Camper]

Thanks for that info. That explains it pretty well, and now I know that.

[+flask]

you car's speedometer is also pretty closely calibrated to the tires that came standard with the vehicle. you mechanic will be happy to recalibrate it for you based on the tires you're driving now...

 

...if you've money to burn.

[packerjim]

The car has original tires and 500 miles on the odometer.

[+GrateBear]

On the other hand, car speedometers have to overread in order to prevent you from speeding[/b]. At least this is the law in Germany.

GermanSailor

That sounds about as bright as setting your alarm clock ahead so you don't oversleep! I used to do that, until I found myself sleeping 10 minutes late because I knew my alarm was set 10 minutes ahead.

I found that doing the 10 minute thing led to sleeping 20 minutes late, as I did 10 minutes to begin with!!

[+Cowboy Camper]

The car has original tires and 500 miles on the odometer.

 

I'd still trust the GPSr over the car's Speedometer. As others have said, cars can be built with a certain margin of error (on the high side) per design. And as I said earlier, the police roadside gizmos can be 'fixed' by the police to get folks to Slow Down on certain trouble roads. Those things can be set by the user or the manufacturer.

 

The GPSr however, uses different technology and really should be thought of as more accurate. My thanks to Prime Suspect for that information, as his post makes sense and explains just how the GPSr gets the readings that they do. That said, I highly doubt that the car's Speedo is built on GPS technology, so if I were to judge one or the other as Wrong, it would be the speedo in the vehicle.

 

Think about this a sec... If the car had a GPS based speedometer in it, the thing wouldn't work for the first few minutes of driving cause the GPS unit in the car would have to pick up the 6 or so satellites before the needle would even work on your dash. Just my few more thoughts, though I suppose I could be proven wrong on this one. As we all know, Technology is moving ahead a lot more quicker than I'm quite able to keep up with.

[+Cowboy Camper]

After posting the above (thinking only of the hand held Geocache GPSr's) I realized that my Road GPSr actually picks up the satellites pretty quickly.

 

Hmm....

 

Now I wonder why that is so, and am also wondering if GPS tech. IS being used in cars these days for the speedometer?

 

With that, I'll concede that I may well not know what I'm talkin' 'bout here. But my Opinion hasn't changed yet, cause I'm still driving things from the mid to late '90's as far as my vehicles are concerned. Compare those to my later '00's GPSr's, I'll trust them over the speedo in the car or truck.

[+Prime Suspect]

After posting the above (thinking only of the hand held Geocache GPSr's) I realized that my Road GPSr actually picks up the satellites pretty quickly.

 

Hmm....

 

Now I wonder why that is so, and am also wondering if GPS tech. IS being used in cars these days for the speedometer?

 

With that, I'll concede that I may well not know what I'm talkin' 'bout here. But my Opinion hasn't changed yet, cause I'm still driving things from the mid to late '90's as far as my vehicles are concerned. Compare those to my later '00's GPSr's, I'll trust them over the speedo in the car or truck.

Many newer GPS units pre-calculate satellite locations for next several days, which makes getting a lock after start-up much faster.

[+Team Cotati]

The car has original tires and 500 miles on the odometer.

 

I'd still trust the GPSr over the car's Speedometer. As others have said, cars can be built with a certain margin of error (on the high side) per design. And as I said earlier, the police roadside gizmos can be 'fixed' by the police to get folks to Slow Down on certain trouble roads. Those things can be set by the user or the manufacturer.

 

The GPSr however, uses different technology and really should be thought of as more accurate. My thanks to Prime Suspect for that information, as his post makes sense and explains just how the GPSr gets the readings that they do. That said, I highly doubt that the car's Speedo is built on GPS technology, so if I were to judge one or the other as Wrong, it would be the speedo in the vehicle.

 

Think about this a sec... If the car had a GPS based speedometer in it, the thing wouldn't work for the first few minutes of driving cause the GPS unit in the car would have to pick up the 6 or so satellites before the needle would even work on your dash. Just my few more thoughts, though I suppose I could be proven wrong on this one. As we all know, Technology is moving ahead a lot more quicker than I'm quite able to keep up with.

 

Something that I'd really like to know: How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate? I've compared the GPSr speed against three vehicles. 1997 Ford Ranger, 1998 Buick and 2005 Toyota Camry. The difference, if any, is imperceptible.

 

Of course I am using a Magellan unit, perhaps that explains it.

[+coggins]

Speedometers been calibrated to read high is one of those sneaky little car things, much the same way that when the gas gauge shows 1/2 tank you actually have used more than 50% of your capacity.

 

My experience is exactly like yours -- with the speedometer showing 100km/h my GPSr consistently reads 97km/h.

Sneaky, and how does that relate to your car's 100K mile warranty?

[+flask]

 

Something that I'd really like to know: How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate?

 

aside from what your local police officer and/or mechanic have to say about it, it only makes sense for the speedometer to come intentionally calibrated a little low.

 

they don't make it intentionally inaccurate, but like anything else, there's going to be some error. it could be bad for a manufacturer to have a speedometer calibrated too high, so it's best if one is going to err to err on the side of caution.

 

so normal accuracy slop falls on the slow side.

[+Prime Suspect]

Something that I'd really like to know: How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate? I've compared the GPSr speed against three vehicles. 1997 Ford Ranger, 1998 Buick and 2005 Toyota Camry. The difference, if any, is imperceptible.

It's hardly breaking news. Car companies have been doing this for at least 50 years.

[GermanSailor]

The mentionend regulations are not unique to Gemany.

 

Australian Regulation

 

An important distinction of the new version is that the speedometer can no longer indicate a speed slower than the actual speed, and therefore, should provide better protection against inadvertent speeding.

 

And the German regulation: §57 StVZO

 

It basically states that a speedomer should overread no more than 10% + 4 km/h (2,5 m/hr)

 

Hope that helps. Commercial vehicles have to get their speedometers and odometers checked regularly.

 

GermanSailor

[+Team Cotati]

Something that I'd really like to know: How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate? I've compared the GPSr speed against three vehicles. 1997 Ford Ranger, 1998 Buick and 2005 Toyota Camry. The difference, if any, is imperceptible.

It's hardly breaking news. Car companies have been doing this for at least 50 years.

 

How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate?

[GermanSailor]

How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate?

It's just an interesting piece of trivial information. But it makes sense and researching is fun.

 

GermanSailor

[+Team Cotati]

How did you guys come to know that cars are being designed and built with speedometers that are intentionally inaccurate?

It's just an interesting piece of trivial information. But it makes sense and researching is fun.

 

GermanSailor

 

Let me ask: From which source(s) did your research yield the information that substantiates the accuracy of this trivial information?

[+va griz]

Every time I've seen real measurements, be it a test done by any of several magazines or a vehicle I've checked myself, the speedo reading has always ranged from correct, to up to 10 percent faster than the actual speed. (for a vehicle with stock tires and gearing) That doesn't prove that the manufactuer are doing it intentionally, but it seems likely and I can see why they would do it.

 

By the way, the odometers are often off by the same amount. That makes your gas mileage seem a little better than it is.

[+Team Cotati]

Every time I've seen real measurements, be it a test done by any of several magazines or a vehicle I've checked myself, the speedo reading has always ranged from correct, to up to 10 percent faster than the actual speed. (for a vehicle with stock tires and gearing) That doesn't prove that the manufactuer are doing it intentionally, but it seems likely and I can see why they would do it.

 

By the way, the odometers are often off by the same amount. That makes your gas mileage seem a little better than it is.

 

I can share a personal experience. A few years ago, I had reason to obtain a speedometer verification on a dynamometer, on one of the vehicles mentioned above. Throughout the test range, there was no more than a 2mph difference, mostly less than 1mph.

 

Suggesting that auto manufacturers are intentionally designing and building vehicles in such a manner, is to me, simply not credible. The chaos in traffic courts alone would be enough to prevent such a practice and there is no rational business reason to do so. I mean, someone on the management team at every auto maker would have to know about this. In my mind it simply defies belief.

 

And to fraudulently and knowingly create a system that 'enhances' gas mileage? I do not believe it.

 

I need to see this 'research' result.

Edited December 27, 2009 by Team Cotati

[+Team Cotati]

I had my new Garmin Etrex H in the car with me yesterday and I noticed the speed reading on the GPS was approximately 2 miles slower than my cars` speedometer . Question is the GPS more accurate or the speedometer? I do notice though on those speed reading gizmos the police here in town put out off the road that my speedometer is pretty much a match .

 

I'd speculate that since the road side courtesy radar gizmo and your auto's speedometer are a match, that in your case, the speedometer is equally accurate with the gpsr. This has been my experience also.

 

It is possible I suppose that the radar gizmo has been calibrated to match the built in error from the auto manufacturers. In that case I suppose that they'd both be equally inaccurate.

 

+ or - 2 mph, of course.

Edited December 27, 2009 by Team Cotati

[+Prime Suspect]

I had my new Garmin Etrex H in the car with me yesterday and I noticed the speed reading on the GPS was approximately 2 miles slower than my cars` speedometer . Question is the GPS more accurate or the speedometer? I do notice though on those speed reading gizmos the police here in town put out off the road that my speedometer is pretty much a match .

 

I'd speculate that since the road side courtesy radar gizmo and your auto's speedometer are a match, that in your case, the speedometer is equally accurate with the gpsr. This has been my experience also.

 

It is possible I suppose that the radar gizmo has been calibrated to match the built in error from the auto manufacturers. In that case I suppose that they'd both be equally inaccurate.

 

+ or - 2 mph, of course.

Those road-side displays are not of the same precision as a true radar gun. I doubt they're ever calibrated once they leave the factory.

 

As for the car speedometer settings - Many years ago, I owned a Chevy with the 1st generation cruise control. It had 1 button, that either set or released the speed. There was no on/off switch as they have today, which means the transponder was on whenever the car was started. The transponders had a relatively short life, and you could count on having to replace it every few years.

 

The local specialist in this was a company whose main business was speedometer calibration. They mainly did this for police cars and street racers. While having a transponder replaced, I asked a mechanic if what I had heard my whole life was true - that auto manufacturers routinely set the speedometers to read higher than actual. He said he had never seen a car under $100,000 that shipped with an accurate speedometer, and all of those read on the high side.

[GermanSailor]

 

Let me ask: From which source(s) did your research yield the information that substantiates the accuracy of this trivial information?

Yesterday I posted this. In this post I added links, because I think it's just more reputable to provide the source of your information.

 

GermanSailor

[+va griz]

The following is fron this article from Car & Driver magaine.

 

http://www.caranddriver.com/features/02q2/...al!-feature

 

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph

 

Also interesting is out of over 200 cars tested at a true 70 MPH, 3 read less than 69, and 90 read more than 71 MPH. So (if you believe them, I do) the good news is speedometers in newer car are pretty accurate, but they are optimistic on average. They said tire wear from brand new, to ready to replace, made about a 2 MPH difference.

[Karaganis]

I can tell you that in my Chevrolet Avalanche, the speedometer is hand calibrated, and therefore is subject to human error. I, along with most other people who drive 2003 - 2007 GM trucks had a speedometer issue in which the stepper motor would fail causing any one of the gauges on the cluster to fail (but was usually the speedo and / or tach). Anyway, the gm guy said that they are hand calibrated when they put the needles on after the motors are installed into the board, and as such they vary.