Speedo vs GPS

[slofox]

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.

[avgas]

Don't trust the GPS speed - the algorithm for its calculation is a 'best guess' method rather than raw data.
It can't use raw data due to update times.

What brand gps is it?

[slofox]

Don't trust the GPS speed - the algorithm for its calculation is a 'best guess' method rather than raw data.
It can't use raw data due to update times.

What brand gps is it?

Yep. Like I said - it is an estimate to give me some idea...

GPS was a tomtom start...

[neels]

I did the same the other day.

On the bike the speedo is about 8k's fast at 100, which is about the same error I get from those roadside speed signs. The car was about the same error as well.

I had suspected the old scooter was a bit out, given that a chinese scooter passed me a while ago when I was doing 30mph on the speedo. Actually it's reading about 1.5 times faster than what it's doing, so at 30 on the speedo it's only doing 20. So it's all good, I'll just pretend it's marked in kph instead

[schrodingers cat]

Accuracy of a GSP depends on its Frequency. Really top end stuff is 20Hz but you can get really good results with 5Hz (sample every 0.2 sec)

I've done a quick search to see what the GPS frequency of a Tom Tom is and can't find it (easily) but I suspect it is 1 Hz or worse. Under hard acceleration or braking it will lag.

Quote from a GPS data logger site:
Speed Measurement. While speed is probably the most important parameter that anyone wants to measure using the data logging system, it is also the most inaccurate in a "conventional" system. The normal way to measure speed is to simply attach a pickup to a wheel to detect how fast it is rotating - but the rolling circumference of a tyre changes by 4% just with wear and temperature. Even worse, the error increases significantly under race conditions where the tyre is under load - typically the tyre slips by upto 20% under hard braking going into a corner. Measuring speed using GPS is now common practice in high-end systems - under typical conditions speed error is well under 1%!

This is true of cars but moreso with a motorcycle as the rolling radius of the tyre alters as you lean

[p.dath]

Don't trust the GPS speed - the algorithm for its calculation is a 'best guess' method rather than raw data.
It can't use raw data due to update times.

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

[sinned]

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

The GPS check of a speedo is as accurate as your ability to maintain a steady speed and the inherent accuracy depending on how many satellites the receiver is locked on to. The top end GPS units display the accuracy as +-distance. If you can be bothered the maths can be done.

[p.dath]

Don't trust the GPS speed - the algorithm for its calculation is a 'best guess' method rather than raw data.

I'd trust the GPS reading more than the speedo reading ... the speedo has no way of self-calibrating.

[slofox]

The GPS check of a speedo is as accurate as your ability to maintain a steady speed and the inherent accuracy depending on how many satellites the receiver is locked on to. The top end GPS units display the accuracy as +-distance. If you can be bothered the maths can be done.

Ok. We had six satellites. I took the 100km/hr reading on a looooong straight on Kaipaki Road for those who know it. I kept as steady a speed as I could. (Not perfect, but over time etc etc...)

All I wanted was an indication. Which I got. 109 on the clock was around 100 on the GPS. Better than guessing which is what I have been doing up 'til now.

Incidentally, I somehow got the idea that the GPS was +/- 2km/hr...according to the joker who owned it I think...

As p.dath says, I'd trust the GPS more than the speedo...

[cowpoos]

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

Totally agree!!

[sinned]

Speedo reading 9kph fast is about right for a Suzuki; my check on 2 Zooks indicated 8kph. Your measurement technique is probably good enough to be relied on.

[vifferman]

The GPS (TomTom) consistently indicates the car's speedo is 2% optimistic at 100km/h. Good enough for me. Oh - that's when the car's speedo's actually working. About 60% of the time, it consistently reads 0.
I checked the bike with the TomTom back when I was running lowered gearing and a speedo corrector. My guesstimate for the speedo corrector's settings of -12% involved bad maths, so it turned out the speedo was under-reading by a few percent. Apparently 6% lowered gearing and 6% inaccuracy (based on polling other VFR owners) doesn't add up to 12%. Well... duh!

[schrodingers cat]

Incidentally, I somehow got the idea that the GPS was +/- 2km/hr...according to the joker who owned it I think...

With a 5Hz reciever the accuracy is 1% so +/-2km/hr @ 100kph ish is believable for a 1 hz reciever.
At (near enough to) constant speed I'd trust it.

[Filterer]

With a 5Hz reciever the accuracy is 1% so +/-2km/hr @ 100kph ish is believable for a 1 hz reciever.
At (near enough to) constant speed I'd trust it.

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

[Padmei]

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?