Speedo vs GPS

[MarkH]

Measuring speed using GPS is now common practice in high-end systems - under typical conditions speed error is well under 1%!

This would be true. Even if a GPS can only give your location within 10 metres it can still give an accurate speed reading, it only requires consistency from one reading to the next, not accuracy on each location reading. A GPS is crap at showing the changes in speed in real time, but travel at a pretty constant speed and they are probably reading correctly to the nearest 1 kph.

[schrodingers cat]

How is update time in anyway shape or form related to accuracy at a constant speed?

Because speed = distance over time

The unit has a built in clock (Time)
The GPS co-ordinates provide distance as a differential between 'plots'

The refresh rate is critical if there is acceleration/decellaration. 5 samples per second ensures accuracy of around 1% A lower rate obviously gives a different 'average' (mean)

You are correct in saying that if the speed is constant then the refresh rate is irrelevant. Personally I'd like a speed calculation from 20 samples not 4 (5hz vs 1Hz)

[schrodingers cat]

This would be true. Even if a GPS can only give your location within 10 metres it can still give an accurate speed reading, it only requires consistency from one reading to the next, not accuracy on each location reading. A GPS is crap at showing the changes in speed in real time, but travel at a pretty constant speed and they are probably reading correctly to the nearest 1 kph.

I find that some systems (designed in the northern hemisphere) have 'issues' from time to time down under. I would suspect that satillite flight paths are more interested in good coverage for the bulk of the world land mass.

GPS co-odinates can 'jump' when the unit has to 'drop' one of the satillites it is triangulating off and pick up another. It is funny to see the jump on track maps during a session...

[MarkH]

I find that some systems (designed in the northern hemisphere) have 'issues' from time to time down under. I would suspect that satillite flight paths are more interested in good coverage for the bulk of the world land mass.

GPS co-odinates can 'jump' when the unit has to 'drop' one of the satillites it is triangulating off and pick up another. It is funny to see the jump on track maps during a session...

Sure, a 'jump' will throw out the speed readings, but unless the unit is constantly 'jumping' then the speed value that it is mostly showing when you are travelling at a constant speed WILL be correct. If generally you get a reading of 100kph on the GPS when travelling at a steady 108kph on the speedo then you can be fairly sure that 100kph is the real speed when you are going 108kph on the speedo. If occasionally you get a reading of something else then that could be a 'jump'.

My experience: readings on the GPS are VERY consistent when travelling at a steady speed in a straight line and not changing elevation. From this I have a good idea of where the speedo needle should be if I don't want a speeding ticket. While accelerating or braking I wouldn't bother using the GPS reading of speed for any idea of my actual speed.

[miloking]

For what it's worth - just in case anyone is wondering...

I ran a GPS on The Vixen this morning to check speedo reading against GPS speed readout...

Essentially, the speedo is about 9% optimistic against the GPS - i.e. at 100km/hr on the GPS, the speedo reads 109km/hr.

This is an average, since there was some variation involved - a bit of lag on the GPS and a tendency for it to read the same speed over a speedo range of 2-3 km/hr. Probably due to lag but also to GPS accuracy as well - maybe..

Sufficient to give me a better estimation of how fast I can go on the speedo without straying into ticketing territory.

Iam getting speedo healer for mine because i changed the sprocket so iam sick of guessing, especialy if they finaly decide to get rid of the tolerances for good (few more public holidays with low road toll i reckon)....maybe worth the idea for yours too (they are about $114USD from egay)

Edit: actualy there is another alternative and thats something called speedo tuner, seems to be way cheaper then speedo healer ($84) and from reviews it seems to be easier to setup....proably worth the money if you dont like doing the math everytime you go for ride.

[onearmedbandit]

When I've compared GPS vs speedo I've noticed something other than a discrepancy between the two, and that is that the difference between the two is always the same when held at a steady speed, so I'm quite confident that my GPS is tracking accurately.

[Max Preload]

...a tendency for it to read the same speed over a speedo range of 2-3 km/hr. Probably due to lag but also to GPS accuracy as well - maybe..

It's called hysteresis. They intentionally do it so the retards don't stare at it watching the numbers changing and drive off the road.

[LBD]

What I've always wanted to do is to find a friendly policeman at the end of a long straight with a speed gun & do a couple of tests. I would like to ride consistently at 100k on the GPS, then with 100k on the speedo & see what it reads on his speed gun. No matter how good your GPS is, ultimately if there is a descrepency between his reading & your GPS then you'll be gettting a ticket.
I don't think an argument that your GPS is top of the line will go down too well.

Anyone tried it?

Yup twice.......and I have the infringment notices pinned to my camp wall s a souvenir....Radar said 115....lights came on and I looked down and gps was 114...on a long straight south of levin....buggar...$80

Kumara straight one week later119 on GPS and 119 on the notice.....$120....Double buggar

To make matters worse....my radar detector mount broke and was lost the week before the first booking....tripple buggar

From experience and talking to the dealer, Ducati speedos read about 8 % optimistic....108 on bike speedo is a neat 100....Which makes the 200 on the race track a dismal 180 ish

[NZsarge]

Essentially, the speedo is about 9% optimistic against the GPS - i.e. at 100km/hr on the GPS, the speedo reads 109km/hr.

That's pretty much the same as my ZX14, reading 109 as opposed the gps at 100. Using a Garmin Nuvi 500.

[avgas]

If you are travelling at a constant speed then the GPS speed should be accurate.

And direction, and altitude.........

I can get GPS time sync down to Sub-uS on structures that aren't moving.
What most cheap GPS units do is window sampling when calculating speed. Depending on what data sits in the window buffer will determine the speed.
But then again its a case of what you are using if for and GPS does what it was designed to do perfectly. The Mil-spec crystals are good for about 1m. I believe the commercial ones are good for 2m.

But yes if you travel at constant speed, with constant direction at a constant altitude - you accuracy CAN be a good as the freq used. Exponential decay outside those parameters.

[Max Preload]

And direction, and altitude...

Only direction. Your altitude is a function of direction. You need constant velocity, the vector quantity, as opposed to constant speed, the scalar quantity.

[p.dath]

... Ducati speedos read about 8 % optimistic....108 on bike speedo is a neat 100....Which makes the 200 on the race track a dismal 180 ish

Assuming the error in your speedo is constant with speed ... maybe the error gets worse the faster you go ...

[onearmedbandit]

Assuming the error in your speedo is constant with speed ... maybe the error gets worse the faster you go ...

On my gixxer the error reads at a constant 13% error at any speed. (Down 1 on the front, std gearing is 8% error).

[Jantar]

I now have 2 GPS units on my bike displaying the speed. My radar detector has a GPS built into it, and I have a Garmin for recording. Both GPS displays are always within 1 km/h of each other when accellerating/decellerating, exactly the same at constant speed and around 8% slower than indicated on the speedo.

[Reckless]

Seems like everyone is searchng for a more accurate speed reading, GPS or speedo!
I imagine the thoughts are if the cops are going to be ticketing down to 104k then we don't want to loose another 8% in speedo error!

To take another angle
The person your following thats doing 80k and is so lacking in confidence they won't go faster because they've been told Speed KILLS is actually doing 73.6k. LOL!!!

You just gotta laugh its all quite funny what the boys in blue are creating! This scenario must be safer surely we are all going slower now?? LOL!!