Speedo Wrong by 7mph

[Dorset Bob]

Originally Posted by Phil-T4

Do you have a military spec GPS as these are the ONLY ones that are accurate

Point taken. I actually had 125.50 seconds, and rounded to the nearest second, but how accurate is my thumb?

The point I was trying to make was the difference in the three readings. They were all done under the same conditions.

I was expecting an under-read on the GPS. I am a bit of a sceptic of these devices, probably by using GPS some 20 years ago, in aircraft, when the Americans used "Selective Availability" and deliberately degraded accuracy for civilian use. This could put you way off course, if you trusted it.

My GPS is never going to be accurate enough to put a missile through a front door, but I think in the conditions that I used (straight, level road) it was a reasonable datum to use to measure the other readings.

On the twisty, hilly, tree covered roads around Dorset though, I am yet to be convinced. I think I will stick to my speedo.

[Phil-T4]

Sorry for the late reply, the gps system has 2 frequencies that are used, L1 and L2, your satnav device will be using the L1 frequency and can be off by 10 meters.
Military devices use both frequencies, comparing, calculating and are accurate to a few centimetres.
The point beginning is you can't compare when all have inaccuracies.

[kaiser]

Even if we accept your accuracy of 10 meters, the chances are slim, that ongoing readings will have deviations that oppose each other.

If you travel at 100kmph, the speed you cover is 100000 meters per hour, or about 30 meters per second.

Your claimed accuracy is thus equal to the distance traveled in 1/3 second.
If all readings are out by the same amount, it will not even affect the speed result at all.
It is of no significance in the scheme of things here, especially not at steady speed.

The GPS signal will be accurate to within one percent under such conditions, typically less than that.
But in stead of debating, grab your stop watch and go out and make some readings as I have done.

[Union Wheels]

Hi Phil-T4,
you say that the position could be as much as 10 metres out, but do you know how much this would affect the MPH read out?

That is to say what is the frequency of the positioning system in relation to the speed, does it check every 10 metres; or 20 metres; or 30 metres; or 40 metres etc?

Just interested to see if it would make a significant difference to the speed read out as you seem to be well clued up on satnavs.

Pete.

[Dorset Bob]

Thanks Phil, I would agree with this if I did not do a speed and distance calculation. However, I measured 2.1 miles in 126 seconds which equals 60mph.

If the error was 10 metres out, that would be in terms of Global Positioning, rather than speed and distance calculations. Unless, for example, there was a 10 metre error that changed to a 1 metre error during those 126 seconds

Edit: I'm too slow with Kaiser and Union Wheels above.

[Phil-T4]

Guys, I have no definitive answer to this, there are too many variables and what if scenarios.

The only way to truly get accurate readings is to physically measure the distance and use laser timing to measure how long it took.

Personally i dont care if my speedo is few MPH off!

[kaiser]

Originally Posted by Phil-T4

Guys, I have no definitive answer to this, there are too many variables and what if scenarios.

The only way to truly get accurate readings is to physically measure the distance and use laser timing to measure how long it took.

Personally i dont care if my speedo is few MPH off!

What is wrong with a stop watch?

[HarryM1BYT]

Originally Posted by kaiser

Even if we accept your accuracy of 10 meters, the chances are slim, that ongoing readings will have deviations that oppose each other.

If you travel at 100kmph, the speed you cover is 100000 meters per hour, or about 30 meters per second.

Your claimed accuracy is thus equal to the distance traveled in 1/3 second.
If all readings are out by the same amount, it will not even affect the speed result at all.
It is of no significance in the scheme of things here, especially not at steady speed.

The GPS signal will be accurate to within one percent under such conditions, typically less than that.
But in stead of debating, grab your stop watch and go out and make some readings as I have done.

The 10metre error is applicable to a single reading, if a series of readings for a single point is taken and averaged, then the accuracy increases. In fact there is software able to do this for you.

If you are measuring a distance between two points using GPS, then the maximum possible error is 20m.

[VMax1000]

I was going to comment but..........

If my watch is off by a few seconds, I might die a day early.

At least the calendar is correct..............I think

Do not have time to worry about the speedo to much

Maybe we can start a thread where speed is linked to mpg.

Something like the faster you go, the more you use...

Maybe instead of mpg we should use gallons/hour.............

[suzublu]

Tend to agree with Hermann here why the need for accuracy in speed I arrive exactly when I want to I don't even look at my speedo unless there are camera's or a law enforcement officer in his tax gathering vehicle