Is Road Angel speed indicator accurate?

[marc777]

My Road Angel registers my speed, and at 90mph on the Speedo, appears to indicate 85mph.

Which is more accurate as I read in the paper this Sunday that GPS based speed is more accurate than the speedo in the car?

Marc

 

[Shude]

The GPS-based speed will be the most accurate. Not only are car speedos set to read "high", they are based on the revolutions of a wheel and tyre combination the diameter of which changes over time due to tyre wear etc!

 

[scotth_uk]

Believe the GPS!

 

[DITTRICH]

..just did a quick calculation (hoping he's done it correctly):-

tyre 17" 225/45 has circumference 2*Pi*r
2*3.14*(225*.45+(17*25.4/2))=1,992mm
after 10mm wear to tread
2*3.14*(225*.45+(17*25.4/2)-10)=1,948mm
difference=44mm

44/1,992 = 2.2%

So 5/90 = 5.6% which seems a little large? Just an observation.

Rgds

Les

 

[glojo]

Every area of England, Northern Ireland and Wales has a calibrated measured mile which is located on a straight stretch of road. (usually dual carriageway) It is denoted by a single black and white banded pole approximately two metres in height. It is there for calibration purposes and is EXACTLY one mile in length. So if you either happen to see these poles or ask your local council highways department where it is (Police Traffic Dept) you can very simply test both.

Drive pass the first post at 60mph and maintain that spped for the whole distance and it should take exactly one minute. Morphine and Mathmatics are not good bed fellows but obviously if it takes longer than one minute your speedometer is on the high side. 1 minute 5 seconds is roughly 55mph. Vehicle speedometers are notorious for being 'optomistic', but are not allowed to read the other way. 65mph roughly 55seconds. 80mph I think is about 45, but my brain is 'well' adled.

Good luck with your search,
John

Aren't our GPS signals given a deliberate degree of inaccuracy by the US?? Obviously only a few metres, but inaccurate none the less.

 

[Shude]

Every area of England, Northern Ireland and Wales has a calibrated measured mile which is located on a straight stretch of road. (usually dual carriageway) It is denoted by a single black and white banded pole approximately two metres in height. It is there for calibration purposes and is EXACTLY one mile in length. So if you either happen to see these poles or ask your local council highways department where it is (Police Traffic Dept) you can very simply test both.

There is one in crossflatts, west yorkshire, it says START OF MEASURED MILE on a big sign, and END OF on another, kinda about a mile further down the road .

Aren't our GPS signals given a deliberate degree of inaccuracy by the US?? Obviously only a few metres, but inaccurate none the less.

Comand is VERY accurate, it counts down to a junction and literally knows when your front tyres have crossed a white line. Comand also uses speed pulse though so it's accuracy isn't neccessarily down to GPS.

 

[Satch]

The Road Angel can lag a little bit in terms of registering speed changes but it is by far the more accurate. Nearly all car makers have the speedo designed to give a slightly "optimistic" reading.

I always go by the Road Angel readout. At 70 mph by the Road Angel car speedo shows about 73/74mph. It has been rumoured that were the Road Angel to show 85mph the car speedo would perhaps show just under 90mph.

In theory, of course.

 

[sym]

My Snooper GPS shows exactly the same speed as my car speedo now at all speeds, - but this is only since I changed to 17" wheels. With 16", the speedo read slightly faster.

S.

 

[Aswall]

My speedo reads a fairly consistent 10% over at all speeds. Make sure when comparing that you do it at a constant speed as the GPS speedos have a slight lag behind real speed due to the sat position only being sampled about once per second

 

[anarchy-inc]

Aren't our GPS signals given a deliberate degree of inaccuracy by the US?? Obviously only a few metres, but inaccurate none the less.

Not anymore.

"Prior to 2 May 2000, the accuracy afforded users of the GPS Standard Positioning Service (SPS) was purposefully degraded through a policy and technique known as Selective Availability (SA). The use of SA gave military users of GPS a position accuracy advantage - one it did not wish to share with potential adversaries. SA was effected by manipulating or dithering the output of each GPS satellite's active atomic clock. This clock controls the generation of all of the satellite's signals and hence the measurements made by a GPS receiver. SA was imposed at a level which would yield a stated SPS horizontal (latitude and longitude) accuracy of 100 metres or better 95 percent of the time for any point in the world during a measurement interval of one day. On 2 May 2000, by presidential decree, the level of SA was set to zero. SPS users immediately saw a quantum jump in positioning accuracy with factors of 5 to 10 improvements. Even a simple handheld receiver can now often yield horizontal position accuracies of 5 metres. When SA was set to zero, the United States Government stated that it had no intent to ever use SA again."

 

[calum]

Would someone please confirm: does COMAND include a speed readout? I have a W211 on order

 

[glojo]

Not anymore.
On 2 May 2000, by presidential decree, the level of SA was set to zero. SPS users immediately saw a quantum jump in positioning accuracy with factors of 5 to 10 improvements. Even a simple handheld receiver can now often yield horizontal position accuracies of 5 metres. When SA was set to zero, the United States Government stated that it had no intent to ever use SA again."

Brilliant news?? I have been bed bound for a number of years and obviously missed this bit of information.. Wreck fishing will be so much easier now.

The GPS-based speed will be the most accurate. Not only are car speedos set to read "high", they are based on the revolutions of a wheel and tyre combination the diameter of which changes over time due to tyre wear etc! .

Does GPS monitor your course over a straight line? If so it will be accurate (especially after the Presedential decree ) on trunk routes, but rural areas will have a very slight in-accuracy, which may make it less precise.

An excellent thread where once again I have learnt something. Thanks to Anarchy!!!!!

Would someone please confirm: does COMAND include a speed readout? I have a W211 on order

I have just studied my COMAND manual and found 'Speed'. I went to the relevant page and it illustrated how to store settings for 'Speed' Dial of the telephone. COMAND is an excellent option, but displaying your speed is not one of them.

Having said that you can have your speed displayed as the default setting option (instead of say temperature). You can also have it show kph instead of mph.

Hope this has been useful. Oh and with COMAND you will also get a manual in DVD format.

Regards,
John

 

[RichardM]

Would the 19" wheels Marc's just fitted explain the bigger difference in speed? When my speedo reads 90mph, my road angel would be 87mph. I've never seen it more than 3mph out.

 

[marc777]

No Richard. This is before I had the wheels fitted.


Marc

 

[Shude]

Does GPS monitor your course over a straight line? If so it will be accurate (especially after the Presedential decree ) on trunk routes, but rural areas will have a very slight in-accuracy, which may make it less precise.

Just like measuring the speed of a vehicle turning a corner using only one wheel as a reference would be

 

[glojo]

Just like measuring the speed of a vehicle turning a corner using only one wheel as a reference would be

Sounds technical to me. I was thinking more like a roundabout. Drive half way round one of those fast at night (not that anyone would)

With GPS would it measure quite simply the diameter and average your speed? (The distance that you have moved over land from point A to point B) Then apply that principal to a windy rural road. Roads in Devon and Cornwall are not renowned for being the straightest in the World (apart from the A38)

Regards,
An inquisative John

 

[Sp!ke]

Put it this way, the 200mph club will not accept a GPS reading as proof since they are known to be up to 10mph out.

That said, they are infinately more accurate than 99.9% of so called calibrated speedo's.

 

[calum]

I have just studied my COMAND manual and found 'Speed'. I went to the relevant page and it illustrated how to store settings for 'Speed' Dial of the telephone. COMAND is an excellent option, but displaying your speed is not one of them.

Having said that you can have your speed displayed as the default setting option (instead of say temperature). You can also have it show kph instead of mph.

Hope this has been useful. Oh and with COMAND you will also get a manual in DVD format.

Thanks John, that's helpful.

A bit of a shame, though. One of the nice things about having a GPS is a precise speed readout, useful for those paranoid about being snapped, so it seems a shame that an expensive satnav system like COMAND doesn't do it.

 

[calum]

Put it this way, the 200mph club will not accept a GPS reading as proof since they are known to be up to 10mph out.

Their speed accuracy depends on the sampling frequency. If it's too large then there will an averaging error as previously pointed out. I don't think they can be said to be "up to 10mph out"? It would depend on the system in use (sampling freq) and also the current speed.

A GPS system can't really show an accurate instantaneous speed measurement, since it has a positional accuracy of around 5m. Assume a once-per-second sampling, at 60mph you travel about 27m per second, and so the GPS could be 20% out. The GPS would only know you had travelled between 22 and 32 metres, and would know your speed was in the range 48-73mph.

So the samplings need to be averaged. This works well since the GPS positional error is evenly spread out, in 2 dimensions in the shape of a circle. So over time the errors cancel and readings become more accurate.

 

[glojo]

A GPS system can't really show an accurate instantaneous speed measurement, since it has a positional accuracy of around 5m. Assume a once-per-second sampling, at 60mph you travel about 27m per second, and so the GPS could be 20% out. The GPS would only know you had travelled between 22 and 32 metres, and would know your speed was in the range 48-73mph.

So the samplings need to be averaged. This works well since the GPS positional error is evenly spread out, in 2 dimensions in the shape of a circle. So over time the errors cancel and readings become more accurate.

Wow!!
Thanks Calum I have just read your message and nipped onto the Internet to corroborate your post. I am getting more knowledgeable by the minute. Accuracy as you have correctly stated was decribed as 5 metres with differential and up to 15 metres non differential????? Using up to 12 satellites. (Is this referring to Anarchy's Presential Decree?)

Thanks again for getting my grey matter working.

Kind regards,
John