Continuing to put the miles on my rebuilt Navigator engine, 650+ now. Thought I would check the accuracy of the speedo against my cycling GPS device.
I taped the devise on to the rev-counter and went for a ride.
Speedo reasonably accurate up to 50mph, about the width of the needle optimistic. Not so good at 60mph, speedo showing ~55mph. Odometer spot on.
All this assume the GPS device is accurate, compared with other GPS's no reason to doubt it. Interestingly, the transient response of the device is sluggish, not too much of a problem on a bicycle, it struggled to keep up with bikes speed!
Speedos are supposed to be…
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Is it chronometric or…
Is it chronometric or magnetic? The chronometric were very precise. That is: accurate as long as they were set up correctly. Magnetic probably not as good. The escapement in the chronometric type is like a clock, and it is as stable as the laws of mechanics. Do magnetic ones lose magnetism? The odometer should stay accurate because it is connected to the wheels by gears, not by magnetism.
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Digital bicycle speedo.
Hi John,
These are the most accurate and if you get a top brand can register speeds of over 150 mph plus have all the other averages, distances and top speed. GPS can be affected by thunderstorms and other atmospheric conditions and as you mentioned, time lag. If you fit a digital one, measure the actual wheel/tyre circumference on a flat surface, not as a Dia x Pi calculation.
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Some thoughts...
Dan F, I wonder if that's strictly true or a by-product of manufacturing tolerances?
Dave C, it's a magnetic instrument, the smaller 60mm diameter version, were these available as a chrono?
Richard C, to be clear the GPS device is not what was traditionally called a "bicycle computer". Its a stand alone device independent of tyre size. That is to say it takes information from satellites like a sat nav. This, I think accounts for the relatively sluggish response while the device processes the data.
When I was working I had access to a Vbox GPS, very accurate, not only speed but position. I ride around the Castle Coombe circuit regularly, my GPS cannot tell me where I am relative to the width of the track, the Vbox device could easily detect position relative to the width of the track. Useful info could be gleaned from it for the fastest way around a circuit, I am told. It could also detect me walking around my small office!
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One of the newspapers had an…
One of the newspapers had an article from someone who used his GPS speedo to squeeze an extra couple of mph when passing a speed camera...and was prosecuted. They blamed GPS delay. So perhaps it's OK to use it to calibrate a conventional speedo.
I used a standard cycle speedo with pickup from a spoke. It worked perfectly up to motorway speeds. It was the cheapest one in Halfords. A cleverer one didn't work as soon as the engine was fired up.
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John Ive copied this from…
John
Ive copied this from another site - The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. If the link works it provides an interesting rabbit hole to dive into!!
The page also explains that GPS is not always accurate especially when going up or down steep hills.
Dan
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EU standard
Dan, had a look at the EU standard, it is indeed an interesting rabbit hole to dive down! I've resisted so far. I am told that the high end software that analyses the data generated by a GPS device will sort out the discrepancies caused by hills. I favour Strava for my cycling data. Low end software in comparison to, for instance, Vbox. Strava will, to some degree compensate for hilly terrain. My test was on flat roads except for the 60mph bit......
The reason for the check is the speedo gearbox, my Navigator has a 2:1 when it should be 19:10 hence the reason for the check. My licence is safe!
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Satelite speed readings
Are based on position change over a time frame based on a FLAT earth. It does not compensate for inclines / declines between those points. The satelite fly's 12000 miles above us and triangulates between the two position but the distance between those points can vary depending on the incline/decline of the road. Dans comment on accuracy is correct.
Richards comment on wheel circumference also correct but remember the tyre pressure and weight of machine and rider. The compression of the tyre affects linea circumference and therefore accuracy.
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Speedos are supposed to be optimistic so as not to encourage riders to speed! - well thats what they are supposed to do now, not so sure about then!