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Speedometer error?

Adrian R

Well-known member
Finally got out for a short, salty ride! Maybe put on 9 miles. The speedometer seemed to say I was going faster then I thought I was. Is there any known error?
 

nielsm

Well-known member
Most speedometers read a bit fast. I’ve found mine is about 8-10%.

The reason is legally they can’t show slower than actual, so most manufacturers target slightly faster to handle variations in tire circumference.
 

Caveman

Active member
I read somewhere that BMW intentionally has their speedos read 4% fast. IIRC something about EU laws are different from USA laws and the speedos cannot have errors that would have the rider going faster than they thought they were. Therefore BMW errs on the side of caution and their speedos read slightly fast on purpose. The numbers actually work on my bike. When the speedo indicates 80 I'm actually doing 77 as being paced by another vehicle. This could all be interweb rumor and urban legend but it's what I've read. I'll try and find where I read it and post a link. YMMV.
 

Lunasea2

New member
Site Supporter
Elite Member
What nonsense! I have a 2021 BMW X3 PHEV along with my 3 BMW bikes including a new R18TC. All 3 bikes have dubious accuracy in speedos, but not the car. So if this has to do with BMW engineers intentionally adjusting the speedo for EU law compliance or tire variances (and to ensure a rider isn't going faster than he may think he is), then why not apply that "logic" (or illogic as I would call it) to the car? Please don't tell me that the car is wrong too. I use a GPS navigator in the car and it is far more accurate than any other device used on bike or car. I do think the speedos are purposely off on the bikes- and it pisses me off!
 

Caveman

Active member
What nonsense! I have a 2021 BMW X3 PHEV along with my 3 BMW bikes including a new R18TC. All 3 bikes have dubious accuracy in speedos, but not the car. So if this has to do with BMW engineers intentionally adjusting the speedo for EU law compliance or tire variances (and to ensure a rider isn't going faster than he may think he is), then why not apply that "logic" (or illogic as I would call it) to the car? Please don't tell me that the car is wrong too. I use a GPS navigator in the car and it is far more accurate than any other device used on bike or car. I do think the speedos are purposely off on the bikes- and it pisses me off!
Dude, don't shoot the messenger. I'm justing regurgitating what I remember reading somewhere. I never claimed it was fact.

Okay, since you asked I am going to tell you that the car may be wrong. GPS data is inaccurate in certain circumstances. Sometimes for innocuous reasons and sometimes intentionally. Things like weather, solar activity, and electronic interference can affect GPS signals and the data sent by the satellites can be intentionally degraded for defense reasons. Unless your receiver is equipped to use ground based data to correct the error your receiver is not 100% accurate 100% of the time. How do I know? I fly for a living and all GPS equipment used for precise aviation have something called RAIM. Here's a link:

https://gps.stanford.edu/research/e...receiver-autonomous-integrity-monitoring-raim

I'm 100% certain that the GPS receiver in your Baby Beemer doesn't use RAIM technology.
 

nielsm

Well-known member
In the US, vehicles must abide by section 393.82 https://www.federalregister.gov/doc...cessary-for-safe-operation-general-amendments

Section 393.82 requires that every bus, truck, and truck-tractor be equipped with a speedometer indicating speed in miles per hour. Speedometers must operate with “reasonable accuracy.” Appendix A to subchapter B (prior to its removal from the FMCSRs on November 23, 1994 (59 FR 60319)) interpreted as “reasonable” an accuracy of plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph). The interpretation indicated that accuracy within these limits is sufficient for a professional driver to ascertain the true speed of the vehicle. FMCSA is including this accuracy limit in § 393.82 to make the requirement easier to understand. FMCSA is also removing the driveaway-towaway exemption to the speedometer requirements because there is no justification for allowing a vehicle to be driven without a speedometer in proper working order. The changes should not result in an increased economic burden on the motor carrier industry.

The rules on motorcycles only specify they must be labeled with MPH or both MPH & KPH.

If we assume 8-10% error rate the 5MPH at 50MPH fits fine.
 

Lunasea2

New member
Site Supporter
Elite Member
In the US, vehicles must abide by section 393.82 https://www.federalregister.gov/doc...cessary-for-safe-operation-general-amendments

Section 393.82 requires that every bus, truck, and truck-tractor be equipped with a speedometer indicating speed in miles per hour. Speedometers must operate with “reasonable accuracy.” Appendix A to subchapter B (prior to its removal from the FMCSRs on November 23, 1994 (59 FR 60319)) interpreted as “reasonable” an accuracy of plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph). The interpretation indicated that accuracy within these limits is sufficient for a professional driver to ascertain the true speed of the vehicle. FMCSA is including this accuracy limit in § 393.82 to make the requirement easier to understand. FMCSA is also removing the driveaway-towaway exemption to the speedometer requirements because there is no justification for allowing a vehicle to be driven without a speedometer in proper working order. The changes should not result in an increased economic burden on the motor carrier industry.

The rules on motorcycles only specify they must be labeled with MPH or both MPH & KPH.

If we assume 8-10% error rate the 5MPH at 50MPH fits fine.
I too have an interest in the accuracy issues and no, not pointing any "messenger fingers." When I said it pisses me off, it applied to an "intentional" adjustment, not to the obvious considerations of tire variance or what have you. As for GPS accuracy, yup, familiar there too. I hold a USCG Master Credential (Captain license) and I am a serious student of navigation relative to GPS variation and I am familiar with "receiver autonomous integrity monitoring" In the vessels I run, we use pretty simple and old fashioned measurements to determine our speed accuracy- the time it takes to cover a measured distance. Is anyone besides me old enough to recall when such signs were commonly posted on freeways for just this purpose? In a boat travelling across an ocean, knowing speed as accurately as possible is crucial in calculating range for fuel issues.

I am aware as well that my car is likely not much better in terms of the accuracy of the speedo, but I've used time/distance calcs to verify (yeah, I'm weird that way) but for different reasons.... I'm a lawyer. Curious about speed as a consideration in auto accidents and liability. Product liability is an area we are especially focused on in my biz.

All that said, the cost benefit to a manufacturer and what is a reasonable degree of accuracy in speedos is fairly regulated (IMO) by US CFR you cite. Same variances apply to bikes as well as my "baby beemer" auto. BTW, what pissed me off was the notion that the bike variance was INTENTIONAL.

Happy riding rubber side down.

BMW R1250GSA, BMW R1200CL and my new fave, R1800TC
 

Caveman

Active member
Found the link I was looking for right here on the forum. I added the 4% part. Don't know how that got implanted in my brain except that when I did the math for the error on my bike it turned out to be 4%.

https://www.r18forums.com/threads/speedometer-mysteries-from-europe.1463/

In thinking about this and the fact my speedo shows a higher speed than actual ground speed there has to be a logical explanation. In the days of gear driven cable actuated speedometers a certain amount of error was assumed. However, in this day and age of ABS sensors and digital gauges I don't see how modern motorcycle speedometers can be consistently off and be consistently fast unless by design. The speeds we see on the speedometer are digitally derived and, therefore, computed the way they are on purpose. It would be very easy to compute the speeds correctly but for some reason they don't. Inquiring minds want to know.
 

tommymck

Well-known member
Dude, don't shoot the messenger. I'm justing regurgitating what I remember reading somewhere. I never claimed it was fact.

Okay, since you asked I am going to tell you that the car may be wrong. GPS data is inaccurate in certain circumstances. Sometimes for innocuous reasons and sometimes intentionally. Things like weather, solar activity, and electronic interference can affect GPS signals and the data sent by the satellites can be intentionally degraded for defense reasons. Unless your receiver is equipped to use ground based data to correct the error your receiver is not 100% accurate 100% of the time. How do I know? I fly for a living and all GPS equipment used for precise aviation have something called RAIM. Here's a link:

https://gps.stanford.edu/research/e...receiver-autonomous-integrity-monitoring-raim

I'm 100% certain that the GPS receiver in your Baby Beemer doesn't use RAIM technology.
Not that it actually matters for this discussion (the speedometer definitely indicates fast), but for the purposes of measuring velocity, I can assure you that your handheld GPS receiver doesn't need integrity monitoring. The velocity measurement emphasizes different observables and the doppler based measurements are more accurate than any wheel-based speedometer. Weather, solar activity, etc. are not corrupting the velocity measurement of any receiver built in the last 20 years, and selective availability as it is currently applied is not a factor. The link that you provided discusses how receivers used for safety critical applications have algorithms that toss out bad satellites from the nav solution in the event that one of them goes on the frits (so that position measurements aren't corrupted). This is a safety thing, but it isn't needed for velocity measurements in any case.
 

Caveman

Active member
You can't accurately measure velocity with GPS unless your position can be accurately determined. The amount of time it takes to move from one spot to the next is how velocity is computed. Velocity and position are not individual components. They're both products of the same data. If one is inaccurate the other one is too.

I'm not using a handheld GPS receiver. I'm using triple GPS units installed in airliners and they absolutely use RAIM technology. It's required by regulation.

Solar does affect GPS integrity. We have weather data specifically for solar activity and when it reaches a certain level it interferes to such a degree that by regulation we can't use GPS as our sole basis for navigation.
 

R12C-R18TC

Well-known member
Premium Member
Site Supporter
Elite Member
enzo5000, I would be very interested to know if you have ever set your cruise at say 70 and checked it against your iPhone speedo simple app?
For once and for all I would like to know how fast I am traveling when the R18 says 70.
 

What is a speedometer?​

The first thing will be to know for sure what a speedometer is. It is a device that measures the rotation of the wheel and the transmission shaft .

This demolishes the first myth that the speedometer measures the speed at which you circulate.

To measure the rotation of the wheel, it makes use of a sensor that is nothing more than a magnet, which is in the gearbox.

It is the integrated electronic system that is in charge of estimating that rotation and therefore marking the speed at which it is traveling on the vehicle's speedometer. Hence there is some difference between the actual speed and the speed we read in the car.

Why does the speedometer mark more speed?​

It doesn't matter what the make of your car or the model, this is something that happens in all of them. The difference is that the margin of error in many cases is lower and in others a little higher. This is because there are some factors that determine the variation in the speedometer with the actual speed of the car.

To understand a little more about the reason for this difference, we have to go back to what the law says . According to the European vehicle type approval regulations, the speed of the speedometer can never be less than the actual speed. There has to be a certain margin of error but upwards.

More precisely, we mean that there must be an upward margin of error of 10% plus 4km/h .

With a practical case, it would be the following: a car going 100 km/h would have a margin of error of 14/km/h. If you see on the speedometer that it says 114 km/h, it will not be like that. The actual speed of the vehicle you are driving is 100 km/h.

What is the law applied to the speedometer called?​

This law is called UN ECE Regulation 39 and it applies when the vehicle is homologated within the European Union.

It makes it very clear that a lower speed can never be reflected on the speedometer.

There are countries such as the United Kingdom where other laws apply since it is not part of the European Union. There the upward law of 10% + 6.25 mph applies.

But it is not only in the European Union that this law exists, but for example in the United States it is also applied. Here vehicles are allowed to have an upward margin of error from 5 to 50 mph.

What factor determines the speed measurement?​

We indicate that the margin of error varies from one manufacturer to another and there are several factors that influence it.

One of the most affected is the type of tire used in the car . A car with new tires will have a larger diameter since it does not have as much wear and every time it rotates, it will be able to have a greater rotation. So the speed in this case will be higher.

With the passage of time, when the diameter of the tire decreases due to the wear of the rubber itself, the drop in air, etc., the distance that will be traveled will be shorter. The car will go slower and there the speed measurement will also be slower, so there is a margin of error.

Is GPS speed measurement accurate?​

All GPS allow you to see how fast you are driving. This works both on GPS that are integrated into the vehicle and on external ones.

For their measurement, what they do is measure the distance that is traveled in a certain time using satellite tracking. With this tracking, they search for the position and calculate the route we have made and divide it by the time it took to make that route.

The result obtained will depend largely on the signal obtained by satellite tracking . It does not depend, as in the previous case, on the tires.

Another factor that affects and makes the measurement with the GPS not exact is that at no time do they take into account changes in vertical direction .

GPS is much more accurate when we are going at high speed since there is more time in the same state. It is the only case in which the GPS measurement can be a little more accurate than with the speedometer.

They don't work as well within a city where we stop constantly. The errors here can be much more noticeable and have information that is not true or that cannot be verified.

It may interest you: The best apps for your car .

So do I have to consider the speed of the speedometer?​

Bearing this information in mind, it is best to drive at a speed slightly less than the speedometer indicates. Thus, you will know that you are really driving at the permitted speed.

Also, consider the external factors that we have discussed and that could affect your condition.

Always comply with the speed limits of each road in order to comply with the law, take care of yourself and other drivers and avoid a fine from the DGT.

Now that you know that the speedometer does not indicate the actual speed, drive responsibly.
 

tommymck

Well-known member
You can't accurately measure velocity with GPS unless your position can be accurately determined. The amount of time it takes to move from one spot to the next is how velocity is computed. Velocity and position are not individual components. They're both products of the same data. If one is inaccurate the other one is too.

I'm not using a handheld GPS receiver. I'm using triple GPS units installed in airliners and they absolutely use RAIM technology. It's required by regulation.

Solar does affect GPS integrity. We have weather data specifically for solar activity and when it reaches a certain level it interferes to such a degree that by regulation we can't use GPS as our sole basis for navigation.
That's just not so. I design these things for a living, including one of the first panel mount GPS receivers PMAd for GA aircraft and what was (I think) the very first WAAS enabled receiver for GA. Velocity is not determined by differencing position measurements. In fact, for most kinematic survey grade receivers, position relies more on integrated velocity rather than the other way around. I referenced a handheld because even for these cheap receivers differenced position hasn't been used for velocity for decades.

Velocity is computed primarily with doppler measurements. The GPS L-band carrier is orders of magnitude smaller than that associated with the 1.023MHz code chip rate, and the measurements are more precise as a result. The computation isn't affected by errors in satellite ephemeris or ionospheric and tropospheric delay effects the same way that pseudo-range based position measurements are. Errors in code-based position measurements don't really enter in to it.

Of course safety critical receivers use RAIM to make sure that they aren't using satellites in the nav solution that are out of whack, but that has nothing to do with the accuracy of velocity measurements.
 

nielsm

Well-known member
I too have an interest in the accuracy issues and no, not pointing any "messenger fingers." When I said it pisses me off, it applied to an "intentional" adjustment, not to the obvious considerations of tire variance or what have you. As for GPS accuracy, yup, familiar there too. I hold a USCG Master Credential (Captain license) and I am a serious student of navigation relative to GPS variation and I am familiar with "receiver autonomous integrity monitoring" In the vessels I run, we use pretty simple and old fashioned measurements to determine our speed accuracy- the time it takes to cover a measured distance. Is anyone besides me old enough to recall when such signs were commonly posted on freeways for just this purpose? In a boat travelling across an ocean, knowing speed as accurately as possible is crucial in calculating range for fuel issues.

I am aware as well that my car is likely not much better in terms of the accuracy of the speedo, but I've used time/distance calcs to verify (yeah, I'm weird that way) but for different reasons.... I'm a lawyer. Curious about speed as a consideration in auto accidents and liability. Product liability is an area we are especially focused on in my biz.

All that said, the cost benefit to a manufacturer and what is a reasonable degree of accuracy in speedos is fairly regulated (IMO) by US CFR you cite. Same variances apply to bikes as well as my "baby beemer" auto. BTW, what pissed me off was the notion that the bike variance was INTENTIONAL.

Happy riding rubber side down.

BMW R1250GSA, BMW R1200CL and my new fave, R1800TC
BMW is a European brand & sells more bikes in Europe than in USA. As a global brand, the bikes need to fit all the regulations in all countries, so they aren't going to do completely different engineering for each market. Cars are higher volumes than bikes, so customizing per market is more tenable. Replacing the background image on the speedo to use MPH as primary is relatively simple. In a digital dash, enabling selecting either KPH or MPH is easy to support. But how the speed is calculated may involve more parts changes between markets, so they will try to make one design that fits all global regulations.

EU rules for speedometer accuracy aren't the same as the US:

The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.3.5. above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
0<=(V1 -V2)<=0.1V2 +4km/h


So any Euro brand is going to build their bikes to meet the intersection of the rules. US says +-10% at 50MPH. Euro doesn't allow it to show less than true speed, so you're left with up to 10% above actual speed being the requirement. As it can't ever show less than true speed, and as tires wear the indicated speed and true speed will change, they will have to be indicating above true speed with enough margin to ensure it never goes below true speed. So in practical terms, it's fair to expect 5-10% higher indicated speed than actual speed on the BMW & it is intentional to ensure they meet all regulations for the markets in which they participate.
 
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