Speedometer speed vs GPS speed

[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]

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.

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!  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

[Bryanj]

I have never yet found a speedo that is accurate and also not one reading low, it used to be a speedo had to be + or - 10% but i believe the - limit has been removed so you cant use it as an excuse

[Mikep328]

"One thing though, the red zone on the new tacho has faded very quickly!  It's new in the pic, but now it is as faded as the original, maybe it is meant to?"

Mine is doing the same.

[Spitfire]

When I first got my F1 on the road I followed my wife through Liverpool, she was in the car and sshe cared me to death with how fast she was going.
as the place has lots of speed cameras.
I was watching my speedo like a hawk, she seemed to be way too fast.
So when we got home we checked my speedo against the car speedo at various speeds, the car was a new Ford Kuga so the speedo was probably a lot more accurate that my 1970's Honda one.
It turned out that mt speedo was 7 mph fast at all speeds, at least it was consistant.

Cheers

Dennis

[TrickyMicky]

Over the years, vehicles, (2 or 4 wheels), that have had many owners, may well have been modified/customised including a change in wheel size, or different profile tyres. All of this will affect the speedo reading. We are not too badly off with our bikes, as the speedo runs off the front wheel, but even here, modern tyres may have slightly different rolling radius. Cars were/are a problem when owners changed gearboxes and final drives.  As someone mentioned earlier, to the best of my knowledge, a satellite system will monitor in a straight and level line, and not account for going up or down hills.  If you feel brave or cheeky enough, you could ask your local friendly ;police traffic pfficer or motorcyclist to follow you for short periods at set speeds. 

[Bryanj]

Follow an english hgv on the motorway, speed limited to 90kph(56mph)

[taysidedragon]

Follow an english hgv on the motorway, speed limited to 90kph(56mph)

😁 Not the ones I got stuck behind a few times today on M5 and M6!

[Athame57]

If you feel brave or cheeky enough, you could ask your local friendly ;police traffic pfficer or motorcyclist to follow you for short periods at set speeds.

Nah....you'll end up getting nicked! 

[TrickyMicky]

Follow an english hgv on the motorway, speed limited to 90kph(56mph)

😁 Not the ones I got stuck behind a few times today on M5 and M6!

Just to help with a common misunderstanding, I think we all agree that virtually all car and motorcycle speedos are not 100% accurate, so it is a regular occurrence when following a vehicle fitted with a tachograph, to accuse them of speeding.  Also, when we are driving our car/motorcycle, we may hold a steady speed to the best of our ability, then , when the road starts to climb gently, we look at our speedo to see we have dropped maybe 3 or 4 MPH and open the throttle to regain our previous speed.  The truck or coach behind you is running on cruise control or speed limiter, and as soon as the speedo starts to drop, even minutely, it allows more power to the engine, leaving us declaring "that cheeky git has just accelerated past me".  When I first drove a coach with a limiter, it  was a peculiar feeling at first when you felt more power being applied at the very beginning of an up-slope, this would happen far earlier than if you had been driving "manually", and the speedo did not move at all.

[Bryanj]

Plus all trucks for the past 3-4 years have to have the auto braking if you get too close, lane nagging and most have a gps type cruise control which eases the throttle at top of an incline and puts more n climbing one

[Mikep328]

Went out yesterday and, during the ride, checked the speedo vs the Tach using road test data from 1975 showing 11 MPH per 1000 RPM in top gear. At an indicated 60MPH, the tach showed 5000 RPM.  SO the actual speed, assuming the gears/sprockets are OEM spec (yes) and the tires are OEM size (yes) was 55 MPH. 

Of course, whether the tach is accurate is another question!