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Speedometer speed vs GPS speed

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Mikep328:
When I was out today I decided to check the speedo indicated speed vs a GPS (Speedometer) app on my iPhone.  Generally, it seemed to show that the speedo indicated  a +5MPH difference at all speed ranges from 20 to 70 MPH.  The speedo (and tach) are new DSS items.  I have no idea if the app is accurate and see no way to accurately check either the speedo OR the app.  FWIW, the app has indicated the GWR London to Bristol run as reaching speeds of 125MPH.  Does it?

Nurse Julie:
I usually check mine by riding through one of those illuminating speed triangle thingys that shows your speed. To be honest, I've usually found them quite accurate compared to clock reading on most of my bikes.

florence:
when I go through one of those 30MPH speed things on side of road, when my speedo says 30, the green light thing usually says 28MPH, which is quite reassuring.  Although, they have a limit.  After overtaking a car on an open road which went into a 30, it just said 'too fast' as I was slowing down.  By the time I reached it it had gone down to 30 :)

Athame57:

--- Quote from: Nurse Julie on August 06, 2025, 03:08:22 PM ---I usually check mine by riding through one of those illuminating speed triangle thingys that shows your speed. To be honest, I've usually found them quite accurate compared to clock reading on most of my bikes.

--- End quote ---
That's what I do on my way to Blackheath a few times a week, there is one on Greenwich South St. My speedo always seems to match it too. However I'm using a new pattern speedo I got from DSS I posted about not so long ago.
One thing though, the red zone on the new tacho has faded very quickly!  :o It's new in the pic, but now it is as faded as the original, maybe it is meant to?

exvalvesetdabbler:
The GPS will only be accurate when on a straight and level piece of road.   It's a measurement between two points so doesn't know if the road curves.

Also the assumption is that your tyre profile ( outer diameter) is the same as what the factory intended.

The original Speedo on my bike is about 6 mph high at 30 and gets worse as speed increases. I'm sure it would do 115 mph easily😃. One that I have just rebuilt is about 3 mph high at 40.

If you can spin the input at a constant speed, and time it to clock  a mile, you can calculate the speed.

A couple of Sundays ago I rode past plod hiding under a tree with his hand held speed gun and he didn't bother me. Speedo was reading 35.

Not sure on these roadside signs, my suspicious nature thinks they have been built to a price and a couple mph added to scare folk.

Regards
Dave

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