is your speedo cheating you?

Same here.
Paj odometer is under reading by 5% with larger tyres.
95k is actually 100k when travelling.
So if I have done 260,000k on these larger tyres, do I admit that the car has done 273,000k when I sell it?
Do I admit oil changes are done every 5250k instead of 5000k?

Oldplodder
Physically it has done said distance but Mechanically it has not

Unless you own a 3.0 litre Nissan,,,,,,,in which case,,,,,,,,,,,,,,,,,,,,

Bilbo

That means he must have clocked you at 113 - in WA the cops always take 2km/h off the actual reading to take into account error.......

Darren,
It was in a 100k zone, it is normal for Qld to allow around 10% over on the highway, so would say the 111k was the actual spped they clocked me at.

cheers
The Ape

I'd be a little wary of relying on the speed calculation from a GPS.

The GPS is accurate to a few metres at best. The speed calculation is done from a point to a point over a small time.

If point A is determined, but the coordinates are actually a metre behind you and point B is determined but is actually a metre in front of you, then the calculated speed could be well out.

It may be smoothed and I guess averaged, but it is no more of a reliable figure than your speedometer is.

IMHO
Ken

Most, but not all GPS's are very accurate.
Some do some "rounding" , (eg:my Navman PDA navigator), however a good unit which will give a speed as eg: 99.5 Km/h (like my Lowrance GPS) will be far more accurate in a straight line than any speedo.

GPS receivers do NOT calculate speed from a point to point over time scenario.

From gpsinformation.net :

"GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off."

One caveat is that most consumer GPS units report speed over ground and ignore altitude changes so when the GPS says 100kph going up/down a steep hill you are actually going faster than that as the GPS reports speed in the horizontal direction only.

Was looking at using the GPS as an aid for navigation in a rally car.
Worked out that a GPS only gets a reading from the satellite about once a second, which at 160km/hr is about 60m, or more.
Also found recording a track that the track log showed speeds of over 200km/hr.

Not that accurate.

Actually the GPS is receiving signals from the satellite continuously it just outputs once per second. Tracks that show 'spikes' of ridiculous speeds were more common with earlier (less sensitive) chipsets, I use a SirfStar II or III chipset and as long as the unit has clear reception it doesn't do this.

This means, generally, using an external powered antenna on the roof of the car as most track errors are to do with momentary loss of reception of one or more satellites and even my most sensitive GPS works best with an external antenna.

To illustrate, even with an external antenna the momentary loss of signal caused by passing under a freeway overpass causes the GPS to report a slightly different speed (In my case lower by 2-3 kph)

Thanks for that pjchris, I have sirfII. Son was trying it out when he navigates, but the speeds are too high (100 to 250+ depending on route) and with the interfernece of tree cover, it doesn't work as well as we hoped.

I drive (semi-competitively) through the pine forests and bush around Mount Gambier once a year and with an antenna on the roof it tracks beautifully, never losing lock and we don't get strange speeds in the track log either.

Peter

Thanks,

Might try it with an external aerial.
Bit late for the QRC round in a couple of weeks.
But might be good in the beetle or friends EVO VIII, they don't get much over 220/230.

I use a re-radiating antenna as well. This enables me to locate the receiving antenna where it gets the best "view". I rarely get dropouts and I use the display function in OziExplorer that enables me to watch the number of satellites I am receiving and the strength of the received signal. This is how I can confirm that the dropouts are minimal.

Like the others with a similar system, I get very reliable speed readouts that I have had confirmed by Police radar.

see follow up above

Well put. Of course most of us buy the "consumer" versions

why...The error of a GPS is determined bu singal cloud etc etc etc, over the period of the measurement and distance, this can be taken to being a constant, hence GPS's even in common use are about 0.2 kmh accurate.

I dont have the doppler equivalent but

Looks like all your effort was for nothing, Mike,,,,,,,,,,,,

Just jokes, Mike, just jokes,,,,,,,,,

Thanks for your effort on my behalf anyway.

Take care,

Bilbo