speed

[tonybromley]

does anyone know just how accurate the speed indicator on my tom tom nav thingy is as its different to my car speedo.

cheers...tony

[seanj]

it should be pretty much spot on. the gps is accurate to within 0.1 metre which means in terms of miles per hour, it ain't gunna shift much.

i take it that your speedo doesn't tally up??

same here, although i have a jap import and it was converted from KPH to MPH and it's about 10MPH out.. a common problem on conversions.

also, there is a rumour that many cars have their speedos deliberately adjusted to prevent people going to fast and getting stopped all the time... whether thats the government on a conspiracy thing or the car manufacturers just trying to stop their cars being listed as the most pulled over?? or it's a total load of twoddle - like most rumours...!!

[Alicatt]

Originally Posted by seanj

also, there is a rumour that many cars have their speedos deliberately adjusted to prevent people going to fast and getting stopped all the time... whether thats the government on a conspiracy thing or the car manufacturers just trying to stop their cars being listed as the most pulled over?? or it's a total load of twoddle - like most rumours...!!

It is a requirement for cars in Japan to have their speedos set 5% fast ie. indicating 52.5 mph on the speedo true road speed of 50mph. You will find that most speedos read between 5% and 10% fast.

[djohnson]

I've got a Nissan Almera (Japanese car). The speed is always 6 mph faster than what Tom Tom reports. It's also 6mph over when I drive a Toyota RAV4.

Although I still think this is the Navigator, not the car. Why I believe this is because I don't always get a good GPS signal (I think I need an external antenna). If I had a good signal then I'm sure I'd get a more accurate speed indication.

[iaveight]

Talked about this under GPS....Your speedo is fine its the unit itself that cant keep up. The faster you go the more error there could be. Far as i am concerned the speed function is just a nice little toy to give you a rough idea when you will arrive to your destination. It has nothing really to do with the amount of sats its picking up or anything else.

[Aceman88888]

Im not sure if your comment is correct. Ive got a Snooper S6 unit as well as my GPS tomtom and the speed showing on this (which is clearly vital to its marketing) is identical to that of the Tomtom. However my dashboard speed is 5-8mph higher. The Snooper uses its own GPS receiver and Im sure that they would receive some very bad publicity if their harware could be perceived as being less accurate than your speedo. Sure, it lags by between 1 and 2 seconds but when on cruise control its clear to see the effect.

[iaveight]

Speed isnt vital to a gps road map sale, the maps and user interface are. I have used walking gps systems they are accurate because you are usually walking less then 10 mph, I have also used a number of aircraft gps systems ranging from units that cost 1000 pounds all the way up to advanced models that are 100s of thousands of pounds and let me tell you there is a huge difference in accuracy. Now we are talking of a software program that runs inside a tiny pda, with the thing on the dash not really being a receiver but being an antenna, the accuracy wont be there. So its 5 or 10 mph off, make no real difference to your time unless you are traveling on a very long trip with no traffic lights or having to slow down for road problems....Speed that is shown on them is a toy as far as i am concerned a fun little thing to watch and give a half decent estimate of what time you will be home so the tea will be nice and hot when you arrive.....

[Semitone]

iaveight

I think you are confusing the speed readout with the time readout. The speed is extremely accurate and in fact is better as you go faster. You can see this from the fact that it will sometimes report a speed even when you are stationary. The time is known very accurately because it comes from the atomic clocks in the satellites and so if you move a larger distance between samples (i.e you go faster) the average speed calculation gets better since the position error is about the same. There will also be a position error caused by the latency of the data (it is only read once per second) but this is constant for a given speed and so will not affect the speed calculation. This error gets smaller the slower you go.

The time to reach the destination can only ever be an estimate because the computer can never know exactly how fast you will be able to go. I have found that on long journeys the times have been incredibly close. I have done several 2-3 hour juourneys and arrived within 10 minutes of the time that it said at the start, so i would say that the estimates are fairly good.

In my car the speedo has an almost constant offset of 1.5MPH from 30MPH, but in my bosses car it varies and by 80MPH on the TomTom the speedo reads 86MPH.