Measuring Speed re Speedometer +/-

Flyboy152

JJ_Egan wrote: »

So its a rolling road needed then .
Dont think i will bother let it get worse and put it under warranty .


jaydeeuk1 >>the word is not mormon but very very close to it .


jje

What is the point? As soon as your tyres wear a bit, your speedometer will be overreading again.

Flyboy152

james2012 wrote: »

I have a Micra and was always being overtaken on the motorway even although I was hammering along at 75 mph according to my speedo.

Then I got a sat nav and discovered my speedo was reading 10% less than the speed according to my speedo.

If your speedometer was underreading, that is an offence.

So I started to drive at 80 mph on the motorway.

That is a bit of a silly thing to do, don't you think?

I still get passed.

That is more of an indication of general driving standards, than your speedometer.

There is one of these radar signs that say too fast if you are going too fast, that has appeared on a road near me when the speed limit changes from 40 to 30mph. I only set it off if I am going 33mph or faster according to my speedo and my sat nav reads 30mph.

The speed indicator devices are probably set at thirty three miles per hour.

Flyboy152

JJ_Egan wrote: »

It won't get worse. It'll not be covered under warranty either.

No surprising that but it will be checked under warranty ,

Driven thousands of brand new cars and never had one so noticeable as this including a few with no speedo .Had a few bounce at 140mph though .
But as the question was an easy method of testing without a rolling road and that's been answered as a no thats all folks must go the nice man with the blue light on his car must be lost and coming to ask me directions .

jje

Only if there is something wrong with it. The dealer will, quite rightly, charge you for the diagnostic work if there is nothing wrong.

JJ_Egan

Flyboy152 wrote: »

Only if there is something wrong with it. The dealer will, quite rightly, charge you for the diagnostic work if there is nothing wrong.

I take a brand new car to the dealer and say i think the speedo does not record correctly .

Same as i take the car in and say the radio does not work correctly they do not turn around and say it works thats £50 mate .


Probably send it out for a test drive rather than do any diagnostic work i guess .


jje

reeac

Flyboy152 wrote: »

A satellite navigation system is not an accurate enough tool to measure exact speed.

Can you justify this statement? A marine GPS is accurate to 0.001 minutes of latitude/longitude i.e. about 2 metres and its timing capability needs to be incredibly sensitive and accurate in order to translate satellite signals into position. It recalculates position every few seconds to give a sequence of positions over an interval of time and thus calculates speed over the ground. A car satnav uses the same satellite system.

mikey72

reeac wrote: »

Can you justify this statement? A marine GPS is accurate to 0.001 minutes of latitude/longitude i.e. about 2 metres and its timing capability needs to be incredibly sensitive and accurate in order to translate satellite signals into position. It recalculates position every few seconds to give a sequence of positions over an interval of time and thus calculates speed over the ground. A car satnav uses the same satellite system.

They use doppler shift as well, so they are very accurate.

jaydeeuk1

reeac wrote: »

Can you justify this statement? A marine GPS is accurate to 0.001 minutes of latitude/longitude i.e. about 2 metres and its timing capability needs to be incredibly sensitive and accurate in order to translate satellite signals into position. It recalculates position every few seconds to give a sequence of positions over an interval of time and thus calculates speed over the ground. A car satnav uses the same satellite system.

GPS really isn't that accurate. Hence why this new galileo system is being built.

Garmin for example claim their devices are accurate to 5m.

Say you're traveling at 40m/s (about 89/90 mph).
Between 2 points at 5m accuracy in time of 2 seconds (about the interval mine measures speed), the gps system could think you've travelled either 70m or 90m in 2 seconds, ie between 78 and 100mph!

And taking the example of 2m given in a marine environment, the same 80m distance could be read as 76m or 84m (85mph or 93mph)


Not that accurate really is it? Granted thats an extreme example, but its based on manufacturers accuracy figures.

The US Govt use a more accurate system for their cruise missiles, and they're known to go astray by 25 m or so on a target!

mikey72

GPS don't do speed by distance.
They know the satellite frequency, they know where it is, and the doppler shift on the received signal equates to the speed.

page 6

“an average of 10 consecutive 1-second Doppler readings
could produce an average speed over 10 seconds with accuracy
of 0.044 knot and 95% confidence”

http://nujournal.net/HighAccuracySpeed.pdf

Flyboy152

JJ_Egan wrote: »

I take a brand new car to the dealer and say i think the speedo does not record correctly .

Same as i take the car in and say the radio does not work correctly they do not turn around and say it works thats £50 mate .

Yes and blooming right as well. No dealer will provide labour free of charge. If you take a car in that needs extensive diagnostic work done for no reason, they will charge you. Non-faults are not covered by warranties.

Probably send it out for a test drive rather than do any diagnostic work i guess .


jje

And how would that measure whether the speedometer was working accurately or not?

Flyboy152

reeac wrote: »

Can you justify this statement? A marine GPS is accurate to 0.001 minutes of latitude/longitude i.e. about 2 metres and its timing capability needs to be incredibly sensitive and accurate in order to translate satellite signals into position. It recalculates position every few seconds to give a sequence of positions over an interval of time and thus calculates speed over the ground.

Yeah and the best of luck trying to get one of them to attach to the windscreen of a Citroen C3. :wall:

A car satnav uses the same satellite system.

No it doesn't. What planet are you on? The average car satellite navigation system uses a far inferior systems to that which you have described above. It does not, for example, take account of vertical speed.