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?
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Dude, don't shoot the messenger. I'm justing regurgitating what I remember reading somewhere. I never claimed it was fact.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!
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.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.
GPS is quite accurate most of the time and it's generally more reliable than speedometers.I use an iOS phone App called "Speedometer Simple". It's GPS based and quite accurate.
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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.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.
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.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.
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.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