Speedometer vs TomTom

[andrewaaa5]

I got a Fabia 2001 model:

When my speedometer shows I am doing 120KMH, my TomTom shows I am doing 110KMH

Is the TomTom more accurate?

[Bigw2069]

Yes.

HTH

[maverick_swe]

A GPS is always more accurate. My Octy has a speedometre fault of +7-10kmh all the way.

[gillywibble]

Because GPS triangulates your position, it is far more accurate than a speedometer.

[devonutopia]

GPS is basically your actual speed.

[postmanpat]

Yup speedo reads 74/75 for an actual 70 (mph) in mine

[Duggie]

Can manufacturers are allowed to factor in a decree of innacuracy as long as it reads high and not low, most cars will do this, my Fabia reads +5mph in most conditions.

-Duggie.

[andrewaaa5]

ok, cheers for the replies. I thought it was something to do with the prevention of 'speeding tickets' etc by the car companies, by giving a slightly 'under' reading on the Speedometer... cheers.

[ScoobyChris]

Originally Posted by gillywibble

Because GPS triangulates your position, it is far more accurate than a speedometer.

But GPS has to take an average reading for speed and can't measure instantaneous speed that well. It can also be easily confused imhe. A speedo can be accurate (if correctly calibrated), but manufacturers build in a margin of error.

Chris

[andy2407]

My understanding is that he GPS/Speedo differential is more likely to be a % variation than an absolute (eg +5mph) value.

Can anyone confirm this?

[DGW]

The comparative readings for my 2007 Fabia using a TomTom 720 are:

Speedo - GPS
32 - 30
43 - 40
54 - 50
64 - 60
75 - 70

[anewman]

I wonder if the speedo under-estimate is reflected in the mileage. I.e. if your car has 100k miles on the clock and has done only motorway driving at 70mph (for example). Clock displays 75mph so over one hour, if the mileometer matches the speedo it thinks you've travelled 5 miles more. That would make the actual mileage closer to 93,333 (assuming mileometer matches speedo).

[maverick_swe]

Originally Posted by anewman

I wonder if the speedo under-estimate is reflected in the mileage. I.e. if your car has 100k miles on the clock and has done only motorway driving at 70mph (for example). Clock displays 75mph so over one hour, if the mileometer matches the speedo it thinks you've travelled 5 miles more. That would make the actual mileage closer to 93,333 (assuming mileometer matches speedo).

No, the digital mileometer is accurate. It's the speedometer needle who has the built in error.
I don't know what it's called in english, but if you have the function to se your average mpg, current mpg, average speed a.s.o, you can check how much the speedo errors.
Set the cruise control at let'sa say 60mph, then reset the avreage speed display. Lett the car run och cruise control for a mile, and check the average speed again. That's your actual speed.
My average speed displayed is less then 1mph (0-1kmh) wrong compared to my TomTom GPS. With some margin, that's spot on.

[andrewaaa5]

Originally Posted by anewman

I wonder if the speedo under-estimate is reflected in the mileage. I.e. if your car has 100k miles on the clock and has done only motorway driving at 70mph (for example). Clock displays 75mph so over one hour, if the mileometer matches the speedo it thinks you've travelled 5 miles more. That would make the actual mileage closer to 93,333 (assuming mileometer matches speedo).

haha... good point... i'll be thinking of this in my sleep tonight

[Mater]

Ok Just to clarify something for you which ScoobyChris has hinted at.

A GPS is not Accurate in anyway shape or form. It is just [b]more[b] accurate than your Speedo.

I could go complex with this but i wont ill simplify it as best as i can.

Your Satnav searches for signals from 24 satellites that orbit the Earth. once it finds a signal that is coming through over a certain frequency and strength (not all satellites are accessible to the Public) then it attempts to lock on to that signal. Once it is locked on it then continues to search for more signals until it has a minimum of 5 signals. Many people will say it only needs 3 but i can assure you for accuracy you need 5 minimum.

The next bit is complicated. It basically uses complex algorithms to determine the cars location by the strength and frequency of signal from all the satellites it has locked on to. This is done roughly every .5 of a second.

Now for the good bit. Your Satnav is only accurate to within 200m. The Americans own the satellites and that error is put in on purpose as Americans being Americans dont want people to have access to the same accuracy as the military. You will in reality get around 100-150m accuracy hence sometimes your satnav can think your driving on a diff road if 2 roads paralell each other within a close distance.

Now your speed isnt accurate because by the time it has calculated your new position from your last position (which gives your speed over that .5 second accurate to within 200m) then your either accelerating or decelerating going up or downhill etc. so you will find that your speed is prob still 1-3mph out depending upon quality of the receiver in your satnav and the speed at which it can calculate the algorithm. This is also the reason it takes about a second for your speed to change on your satnav when accelerating

The most accurate system is a laser and even that can be flawed but its accurate enough for prosecution purposes.

HTH

Carl

[gadgetman]

Unless your GPS device has been calibrated, then DONT rely on accuracy.

I thought my speedo was approx 10% too high until I got someone with a laser setup to check. Seems MY speedo is spot on!

Had I relied on GPS speed, I'd be risking my licence!

[TKW]

Originally Posted by Mater

The Americans own the satellites and that error is put in on purpose as Americans being Americans dont want people to have access to the same accuracy as the military.

Not for nearly a decade, they haven't. Do keep up

Statement by the President regarding GPS.

[Mater]

Originally Posted by TKW

Not for nearly a decade, they haven't. Do keep up

Statement by the President regarding GPS.

Ok you believe that then. However having been in the Army for over 12 years i know an awful lot about the GPS system as its part of my job and i can assure you that they still have the facility to turn them off to the public if they so wish.

The system you refer to is Selective Availabilty which is only the Degradation of the a Signal and not the complete switch off of the system. Although they have stopped degrading the signal there is still an element of error in them which is around 200m. like i said in reality it becomes around 100-150 depending on No. of satellites you have locked on to.

[said in sarcastic voice with wry smile] So its not me who needs to keep up rather its you who needs to research properly

Laters

Carl

[maverick_swe]

My GPS is not 100-150m off, unless i go faster then 100mph.

Everytime i switch it on, the possition is spot on.

[TKW]

Originally Posted by Mater

The system you refer to is Selective Availabilty which is only the Degradation of the a Signal and not the complete switch off of the system. Although they have stopped degrading the signal there is still an element of error in them which is around 200m. like i said in reality it becomes around 100-150 depending on No. of satellites you have locked on to.

Surely the four current sources of inaccuracy (SA is switched off) are clock drift, ephemeris, Doppler shift and atmospheric shift. Ephemeris and clock drift are tiny, and Doppler shift won't affect a car travelling at < 150mph. The remaining source is significant (the 1 or 200m you mention is plausible) but, over the second or so time gap between each position fix as used to calculate speed this will remain constant so any speed given is quite accurate.

Or have I forgotten another source? It's something I've wondered about before when people mention this topic but have previously never found someone else to ask.....