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Not having the car long I drove for a conference with work from Newcastle to Sheffield and used tomtom navigator 5 on my PDA to find the hotel. I noticed that at an indicated 70mph on the Speedo the satnav recorded 64-65mph.

Is an over reading of 5-6mph usual?
 

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I was really surprised at the difference between cars, at an indicated 65mph in the Golf my Road Angel says 61 but in the Focus I have to be doing and indicated 70mph for 61mph (or similar figures [;)])
 

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I end up following my road angel as a speed guide. My focus used to be a lot worse for the wrong speed but the Golf is still just under 10% out
 

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I have also noticed this and wondered whether the mileage was out as well...

Thinking about it since, I think it is VW playing ultra safe... If they didn't and a customer was caught speeding and claimed they weren't and it was later found that the speedo was out then who would be responsible? Possibly the car manufacturer?

I did read that Nissan has delayed the roll out of their 'auto park' whilst the lawyers get rich figuring out who would be responsible if it totalled a parked car...

Maybe its the same kind of reasoning that make VW play safe?

fM
 

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Mine is 10% out throughout the speed range, even at an indicated 100 mph Im only doing 90 mph according to several sat nav systems!
 

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have you ever thought of this:

i have seen some references to Road Angel being accurate.

i thought they were great, however, they only calculate every so many meters (so they are not completely realtime)

also my mum showed me something she read in the paper the other day, something i had never thought of. .the GPS speeds (such as on snoopers and angels) can be totally wrong. as they measure ground speed, so that means when you go up or down hill the speed will be wrong.

an extreme, but impossible case (just used to explain) is:

a 90 degree hill 1km long, the road angel would show a ground speed of 0mph as you are not moving horizontally

so as you can imagine a failry steep hill, as some are on motorways and country roads, can totally destry your GPS speed rating.

also in the same article in the paper it said (due to the above inaccuracy) that a GPS system cannot be used in place of your speedo being broken or temperarily not working.

hope the above makes sense as i havent read it back through lol

thanks
 

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I would imagine that the GPS engine inside of the road angel will calculate speed quicker than every so many meters. If the the main brains in the road angel take the gps data via NMEA strings then they will probably be updated every 1 second but the actual calculations will be done much quicker. They could however use a host based system whereby the host processor decodes some of the raw data from the GPS hardware and takes the position data diretly. Also when the GPS engine has a 3D fix it is possible to calculate hight above sea level and this can be taken into account when calculating the speed.

Chris
 

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This topic has been discussed many times in many places. To be legal, the speedo must be within the range -0% to +10%. In other words it must never read low but can read up to 10% high. So manufacturers aim to be somewhere in the middle of this range and the speedo will always read high. The distance travelled may not be out by the same amount.
If the wheels/tyres are changed, there MAY be a different rolling radius and if so the speed and distance travelled will both be affected.
It's true that the GPS may read wrongly on a slope, but for most practical road inclines the error will be very much less than the speedo error.
 

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As above, surely a very valid point in how accurate a reading is on the speedo is the size of wheels that the car is wearing and if they are calibrated to the car. eg put 19s on a car that used to run on 15's. An even more interesting arguement might be if the cars are individually calibrated, because Golfs left the factory wearing anything between 14s and 18's, as well as the fact that some cars changed alloys after manufacture at PDI stage, and could have gained a size of two. OK, the sidewalls sizes are smaller on bigger rims so compensate to a degree, but surely it must put the reading (still further) out?
 

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As I said this topic has been done to death and there are lots of references including this:
http://www.raceandroad.com/rollingradius.asp
Most of the error with a bigger wheel is taken out by a lower profile tyre. If you don't fully compensate with thinner tyres then not only will the speedo be wrong but you will lose acceleration due to the longer overall gearing, plus the extra wheel + tyre weight at a larger effective radius.
 

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you wanna know your actual speed?

got climacontrol?

Press Fan Down, Temperature Colder, ECON at the same time

Use the temperature control buttons to cycle through the various codes

(red = higher number etc)

go to 16 and thats your ACTUAL speed in KM/H.

some more...

1 Interior temperature (sensor)
3 Sunlight sensor
7 Coolant temperature sensor
9 Blower speed (adjustable)
10 Measured blower speed (sensor)
11 Defog valve (adj. 0-255)
12 Dashboard ventilation valve (adj. 0-255)
13 Recirculation valve (adj. 0-255)
14 Lower ventilation valve (adj. 0-255)
16 Speed (sensor)

jammy
 

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Not having the car long I drove for a conference with work from Newcastle to Sheffield and used tomtom navigator 5 on my PDA to find the hotel. I noticed that at an indicated 70mph on the Speedo the satnav recorded 64-65mph.

Is an over reading of 5-6mph usual?
Varies from car to car, mine is 3-4mph over
 

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Jammy how's that calibrated and calculated tho?? What's to say it's actually accurate??

C
because everywhere i've read about this "diagnostic" mode has said "actual speed" and when in action it shows a lower km/h than what the speedo says.

therefore i believe it to be accurate.

someone with tomtom test it out?

jammy
 

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Nope not true, it may be more accurate than the speedo speed but it is not calibrated and 100% accurate. It can't be. There's too many other variables.

Read my post in the link mention above. You'll see a difference in reported speed with a new and old tyre, tyre inflation, different makes of tyres actually have a different radius.

J.
 

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To prove my point heres my post in the other thread with some figures.

Not true, it'll be close, like I posted earlier in the thread you need to take into account tyre infaltion, tyre wear, different brands of tyre etc.

For example if a new 225/40/18 tyre has a diameter of 637mm the circumference is 2000mm (I am using pi at 3.14 for these exmaples so not 100% accurate but will give you an idea)

So that tyre has 8mm of tread say. When worn to the legal limit the tread will be 1.6mm so the diameter is now only 622.2mm and therefore the circumference is now 1953.7

The difference as a % between old and new is 2% thats without taking into account for tyre inflation. So if your speedo is overreading by 10% with a new tyre it will be over reading less with a worn one.

Thats not taking into account the different diameters of different brands of tyre when new. Obviously the difference above is very small but if you take into account everything those small differences soon add up.

And yes I have far too much time on my hands!

J.
 

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That's along the lines of what I was thinking as how do you calibrate it? Thanks for popping that up J [y]

C
 
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