GPS Speedo - How accurate?

[David Price]

Gents,

I have recently purchased a Streetpilot Satnav.

When the car speedo says 70mph, the Streetpilot says 62mph.

Can the satnav speedometer be trusted?

Is it a more accurate measurement of speed than the car speedo?

Cheers

 

[Fanum]

Gents,

.

Can the satnav speedometer be trusted?

Is it a more accurate measurement of speed than the car speedo?

Cheers

Yes.

It'll be half a second or so behind your accel/deceleration because of the signal reading gaps, but if you settle on a constant speed, it's far more accurate.

 

[(RIP) Bin Ridin]

At steady speed in a straight line on th level , GPS give THE speed - no question.
I got stopped once by a cop for overtaking him going up a hill on a long bend. He said I was exceeding 60, but dropped it when I showed him the GPS. However, I think he might have been right, and the thing was reading low as it was workingon a series of straight lines (chords).
Not sure if it knows about hills - I think it works on horizontal projected distance, but I am open to correction!

 

[unbalanced]

There are errors involved, and the figure will depend on various factors. Garmin use an avaraging algorithym so your gps continues to give data even though you may not have enough satelites for any accuracy.
Find a long flat straight road (like a motorway) on a clear day and compare the bike spedo at say 60mph with the gps reading.
Time the bike on the same stretch over a mile (use the motorway markers) at a constant 60mph on the bike.

You can then calculate the true speed for the bike, and you will know the error in the bike speedo and also the gps at 60mph.

You need to repeat this at any other speed you want data for - probably 30,and 70mph.

 

[Greg Masters]

Definitively, the GPS gives your EXACT average speed every second.

Assuming that it has a clear view of the sky and is reading the satellites OK, there will be no error significant of mention here.

Greg

 

[BBB61]

So a certain gentleman at the Triumph warranty department is talking out of his rear end when he says that the speed reading of a GPS system is not as accurate as the speedometer fitted to a vehicle then?

 

[GSmonkey]

So a certain gentleman at the Triumph warranty department is talking out of his rear end when he says that the speed reading of a GPS system is not as accurate as the speedometer fitted to a vehicle then?

I'd say so. Although you can get a few metres error with an absolute position, it should be almost bang on re. rate of change of position i.e. speed. As mentioned above, time lag should be the only significant factor to take into account (assuming you're in the open).

Your speedo should always over read by up to 10%, but never under read.

 

[LOLGEOFF]

it should be almost bang on re. rate of change of position i.e. speed. As mentioned above, time lag should be the only significant factor to take into account (assuming you're in the open).

That's a good point by Bin Ridin about going up or down hill though. Actual distance covered compared to horizontal distance must make a slight difference

 

[Stig Of The Dump]

What are you doing tomorrow?

Just a thought, I shall try this tomorrow find a large carpark or small roundabout for that matter drive around it at a given speed and see if it confuses the speedo on the satnav once I have done that I shall go and jump out of a plane (with a parachute) and see if it gives my vertical velocity.
That will give us all something to do tomorrow

 

[LOLGEOFF]

once I have done that I shall go and jump out of a plane (with a parachute) and see if it gives my vertical velocity.
That will give us all something to do tomorrow

It will have to be from a balloon on a wind free day, otherwise you will be carrying some forward momentum from the plane or the wind.

 

[Stig Of The Dump]

It will have to be from a balloon on a wind free day, otherwise you will be carrying some forward momentum from the plane or the wind.

Blimey I will have to learn to run like hell if Im going to have the same ground speed as the plane when I land!!! wish me luck Im doing this you

 

[HMR]

Assuming fair satellite reception, and assuming a speed around 60 mph, you can assume the error in the speed calculated by the GPS to be less than 1%.

The error in the speed shown by the bike speedometer is typically in the range 5-10%. This large error is not due to technical reasons but to legal reasons. The bike instrument is designed to have this error!

 

[Deleted account 220427002]

The error in the speed shown by the bike speedometer is typically in the range 5-10%. This large error is not due to technical reasons but to legal reasons. The bike instrument is designed to have this error!

Why? seems stupid not to have accurate speed indication if possible

 

[HMR]

Why? seems stupid not to have accurate speed indication if possible

Heard about Big Brother? He thinks that you may drive faster with an accurate instrument.

I think the law says that the bike instrument MUST show a speed that is 0-10% higher than the actual speed. Therefore all spedometers are designed to show 5% higher speed than actual speed.

The GPS is still alowed to show actual speed....

 

[DodgyGeeSer]

I got stopped once by a cop for overtaking him going up a hill on a long bend. He said I was exceeding 60, but dropped it when I showed him the GPS.

I got stopped by a member of an Garda Siochana last week. Two fecking penalty points later, instead of showing him the speeds on the GPS, I had to convince him that it wasn't a radar detector.

 

[GSmonkey]

Why? seems stupid not to have accurate speed indication if possible

When you're mass producing something it will have an error of +/- X. So if you're designing a speedo that cannot under read, it will have to be designed to over read so that even the speedo's off the production line that are just within tolerance are still OK. Hope this is clear

 

[Number 6]

Garmin say their Quest is accurate to 0.05 metres per sec, which converts to 0.1118466 mph, how accurate do you need

 

[John Armstrong]

Garmin say their Quest is accurate to 0.05 metres per sec, which converts to 0.1118466 mph, how accurate do you need

Assuming that it has the required satellite reception for that accuracy. not enough ggod accurate satellites means it will still calculate to that level, whether it corresponds to reality

If you want to see what I mean, ride / drive in a bad reception area, eg narrow lane with trees over it, and then see how slow the speed updates on the GPS and how inaccurate it can be, at times.