Checking Your Car Speedo

[+rufty tufty boys]

I know this was mentioned some time ago and there was some debate, but

 

In answer to the question "How can I check the speedometer on my Volvo V70?" the Sunday Times recomend "The easiest way to get an accurate check is with a GPS portable sat nav unit, which have speed read-outs and are totally accurate."

Edited November 1, 2004 by rufty tufty boys

[+Lance Ambu]

Of course it's accurate, if you don't count showing you as driving in the roadside field and performing a 360 when you are stationary.

[kitefan]

My GPS never did that! WTF are you doing with it while you're driving along?

 

Ahhhh!!! Of course! You've got one of the self-wind models

 

Mine gives absolutely consistent results for any given speed on my car speedo. I've found that 90% of cars show a speed 10% higher than you're actually doing! It's a government conspiracy to get everyone to slow down without them knowing!!!

 

On a dual carriageway the speed limit is 70mph (excuse me for stating the obvious but a lot of people think it's 60mph!!!!! numpties) and the police/speed cams will do you if you are going more than 77mph as they give you 10% leeway anyhow. This means that in my car I can do 84mph on the speedo before I have a problem! It feels pretty nerve racking the first time you belt past a police car driving at '70' in the slow lane, or shoot past a speed gun... but I've been doing this for 2 years now and have never been penalised for speeding.

[+The Hokesters]

I brought this up several months ago and the general consensus was that the speedos were accurate but I have to agree with Kitey - I reckon my car is around 10%.

 

Incidentally a police friend of mine said that policy (for most National speed Limit roads) is 10% + 3mph.

 

Si

[+Fangsy]

The clock on my old rover214 seems to be pretty consistent. I've done the same test with the gps giving a speed readout (much safer when you have a passenger holding it and reading the speed out for you) compared to the car's clock. when car says 70, GPS says 66 - 67mph, at 40 on the car the gps reads 38-39.

I found the most consistent readings were when going in a straight line (we have a few roads like that in Wales ) and not accelerating or braking. Probably because the GPS isn't calculating a shorter distance across a curve. I also found the general accuracy of the gps improved when traveling in a straight line in the car, usual accuracy standing still is about 18 feet, in the car I saw it go down to about 12 feet.

[+TheCookieCrew]

Sounds good, I too have used my GPS to correct the speedo on my car, and guess what, If the tyres were correctly inflated then the GPS and speedo were spot on, slightly deflated and the speedo was approx 8% out (over !!), so if you are constantly getting a higher reading on your speedo thatn gps try inflating the tyres to the manufacturers recommended setting, and try again....

 

Not saying it will work for every speedo, but it did for mine.....

 

Mike

[+Lance Ambu]

My GPS never did that! WTF are you doing with it while you're driving along?

 

Ahhhh!!! Of course! You've got one of the self-wind models

 

The GPS was connected to a laptop showing autoroute - as the software had no "snap to" function my position relative to the road was shifted left (or the road was drawn to the right of my position). If I was going somewhere using the GPS "go to" then when I stop the arrow takes the headstaggers.

 

But seriously, I agree that most cars overestimate and that this is a more acceptable error safety wise. There does appear to be a time lag before the gps catches up with the speedo so a steady speed, straight run is preferrable if calibrating the speedo.

 

Working the other way around, I'm sure I've seen reference to speeding tickets being calculated using satellites, in Australia, IIRC

 

PS I adapted the wind up model to run off a roof mounted windmill, which was OK until you had to reverse

[+Renegade Knight]

...I've found that 90% of cars show a speed 10% higher than you're actually doing!...

I've got two vehicles that did that. For my geo-rig the solution was bigger tires. Those put me at 5% over but that's better than 10% under.

 

My minivan is pretty much spot on.

[+Seasider]

On a dual carriageway the speed limit is 70mph...

Only if it's got a central reservation!

 

Cheers!

 

Seasider

[+kewfriend]

 

Think twice before speeding and having your GPS unit running - errrrr ...... the log is pretty precise evidence

 

[ do car GPS units also log ??? ]

[+John Stead]

Only if it's got a central reservation!

A dual carriageway without a central reservation?!!

[+Beds Clangers]

Driving down the road watching Fuwagi maps for an unlogged cache didn't notice the speedo

 

Sorry, was steering with my knees, trying to put the co-ords into my Garmin, talking on my mobile, tipping the last bit out of a bag of crisps, reading the Daily Star (Whoops, Daily Telegraph), trying to pour a cup of coffee and eating a sandwich.

 

The copper who stopped me said Hello, Hello, Hello what have we here then, either your a complete lunatic or a Geocacher trying to beat Seasiders record for caches in a month.

 

Well what could I say, I'm a lunatic?

 

Apologies to Seasider

 

Sorry, what was this thread accuracy of the speedo? didn't notice. 3 points and a £60 fine

 

Nick

Edited November 2, 2004 by Beds Clangers

[+Kitty Hawk]

Beds Clangers - you can use that ticket to bag a cache - Look Here

 

I was booked at 80mph on a dual carriageway with central reservation which I though was a bit miserly. (I was less polite at the time, but the cache eases the pain)

 

Therefore, I would be wary about testing your luck with Gatzo's although I am also happy to speed past a police car - I use the rule of thumb that if they are going slow, they are trying to let you past without causing a jam on the motorway.

[+The Hokesters]

Question for the techies - if the accuracy of GPSrs normally hits about 15 ft at best - how can the speed be accurate?

 

I am sure there is a way but my meagre head can not get round it.

 

Si

[+Kitty Hawk]

I have the same trouble - it must rely on being equally inaccurate all of the time, or else, it can average out the readings - doesn't it give you a "spot" every three seconds?

 

After a reasonable number of spots accuracy will be reached - or over the course of a mile, 15 feet is only 0.2% inaccurate, which at 60mph is about a 10th of 1mph.

 

I don't know if I've stumbled across the answer here, someone else will say.

[kiteflyer]

Just another thought, what happens if you are going up or down a hill? Isn't GPS measured on a flat plain? So using some greek guys theory you would have faster land speed on an incline than what the GPS was reading

I think thats right, anyway

Kiteflyer

[+Cryptik Souls Crew]

it can average out the readings

Thats what it does, your gps receiver will have a smoothing filter to average out the readings.

 

Try driving at a few mph and then stop, your speed reading on the gps won't immediately show zero

[+kbootb]

it can average out the readings

Thats what it does, your gps receiver will have a smoothing filter to average out the readings.

 

Try driving at a few mph and then stop, your speed reading on the gps won't immediately show zero

A car unit (or nav software on PDA) does more than that. It extrapolates your position.

 

Drive into a tunnel. My hand held Garmin reports loss of GPS instantly, but the PDA using satnav has the little arrow carrying on for about 10 seconds before reporting loss of signal. If it didn't do this it would report loss of signal everytime you went under a significant bridge or tree cover.

 

You can also see the effect if you ignore its instructions in nav mode and don't take a turn, It assumes you have made the turn and the arrow goes along the expected road until it has had enough readings that convinces it otherwise and then you will see the arrow leap to the real position.

 

This is done to try and keep the map update as accurate as possible. Because of the delays in processing it actually has to 'think ahead' and work on the position a little in advance of where the calculation actually says you are.

 

Similar methods are used to track fast incoming objects. The software gets a few readings, predicts future points and then compares actual readings with predictions and updates future preditcions. Without this, intercepting by sending your own missile up would be impossible, you need to predict the path.

 

It seems the brain does a similar trick. If you work out the time taken for an image of a tennis ball flying towards to be processed by the brain and the time taken to get nerve impulses to travel to the muscles it would seem impossible to hit or catch a ball. The brain must be doing the predict and update trick.

 

(Whoops, miles off thread, but been talking to a programmer about interception techniques)

[+Pharisee]

A car unit (or nav software on PDA) does more than that. It extrapolates your position.

I don't think it's just car units; I'm pretty sure my GPS III does that too. If I walk in a straight line towards a cache watching the distance count down and then stop when it reaches zero I invariably find that I've gone past the cache by about 30 feet. After a few seconds the GPS III will 'catch up' and indicate that the cache is 30 feet behind me. I always slow down or stop when it shows about 50 feet to go, wait until the reading stabilises and then carry on from there... That method works for me ;o)

[+The Hokesters]

I don't think it's just car units; I'm pretty sure my GPS III does that too.

My Geko too - if you go into a tunnel it can take at least 10 seconds before it reports weak signal - then it asks if you have gone indoors

[+The Spokes]

Just another thought, what happens if you are going up or down a hill? Isn't GPS measured on a flat plain? So using some greek guys theory you would have faster land speed on an incline than what the GPS was reading

I think thats right, anyway

Kiteflyer

With a lock on 4 or more satellites the Gps knows your height as well. I have recorded a speed of 86 mph on skis. 3 runs 77 first time 81 on the second run and 86 on the third run. (I did this as I thought the first run was a false reading ) Now if it was just over a flat plain then this would mean I would have been going a lot faster than that. I have checked it out with my cycle computer as well which is calibrated very accurately. At 25 on the flat it matches the Gps. Down a 1 in 4 hill at 51.6 it also matches exactly. ( I looked at the max at the bottom when I had stopped)

Edited November 4, 2004 by The Spokes

[+wigglesworth]

VW Speeds

If you have climatic control on a VW here is a different aproach. I have tried it and using the approach at the end of the description my Golf GDi worked great.

 

Speedometers have a factory built-in percentage error to show a speed faster

than your actual speed of travel.

 

The Climatronic (Climate Control) system has diagnostic codes that you can

access by the pressing of certain buttons on the Climate Control console.

 

One of these codes shows your actual speed of travel without this built-in

percentage error.

 

These codes are accessed by one of the following procedures.....

 

A/- To access the actual speed display, start your car and then on your

climate control panel press and hold-in both the Econ and Auto buttons or

the Recirculation and Temperature Up buttons. This will change your outside

temperature reading to Fahrenheit.

 

Then press the recirculation button and at the same time press the upward

pointing arrow button above the recirculation button.

 

Your climate control screen will change to a different mode displaying 1c.

 

Use your temperature control + button to display code 17c. This is the

actual speed display feature.

 

Press the recirculation button again to activate the code and to display it'

s reading in mph.

 

(Accessing the codes with the outside temperature reading displayed in

Centigrade will display your speed in kph)

 

To revert your Climate Control to normal mode, simply press the Auto button

or switch off the ignition.

 

Some SEAT's only display the speed in kph whether or not Fahrenheit or

Centigrade is selected.

 

B/- To access the actual speed code on some VW's (Passat's etc) the

following method applies....

 

Hold-in Fan Down, Temperature Colder, ECON all at the same time. Use the +

/ - temperature control buttons to view the various codes. Code 16

(sometimes 17) is the actual speed code.

 

Code 16 displays the speed you are travelling at on your centre screen in

kph.

 

Press-in the Econ and Auto buttons together to change to mph.

 

To revert your Climate Control to normal mode, simply press the Economy

button or switch off the ignition.

 

---------------------------------------------------

REMARK

I did not think this up. It appears to be information from instructionbooks

used by Volkswagen/Audi/Skoda/Seat garages.

[+stu_and_sarah]

The 10% speedo thing is not a conspiracy. Because it's almost impossible to get things totally accurate, a vehicle speedo may read anywhere from bang on to 10% over the correct speed. It must not read under (because then you would be speeding without knowing it).

 

If you always bank on the 10%, the you're being risky because it may be reading accurately, especially since the accuracy changes depending on your speed.

 

Most police forces have a 10% leeway (it's actually 10%+2 in West Mercia as stated in a document which landed on my doormat last week ) but they CAN prosecute at any amount over, so again don't bank on it, particularly in Cheshire

 

So, basically, if your car speedo is reading over, you might be speeding and you could be prosecuted. Usually, there's a lot of slack in the system, and you'll be lucky.

 

As for GPS accuracy, I believe that relative movement is more accurate than absolute position. So while it might only be able to definitely place you within 30 feet, it will probably still know you moved 2 feet on a bearing of 20 degrees. I may be wrong though.

 

Cheers,

 

Stu

[+Haggis Hunter]

I agree with Stu & Sarah, a colleague of mine who is an ex police officer said that just a couple of weeks ago someone got fined for doing 31 mph in Edinburgh City centre, he was in a 30mph speed zone.

 

I personaly go buy what the speedo is telling me. I know that it isn't necessarily a true reading, but I think you can't go far wrong if you go buy your speedo. I would have thought that trusting anything else would have been against the law, but I may be wrong.

[+The Forester]

The recently introduced law which prohibits the use of a hand-held mobile phone also covers hand-held GPS units.

 

It would be a pity to successfully avoid being nabbed for speeding only to be prosecuted for operating the GPS!

[+rufty tufty boys]

Ah but with a bit on velcro the GPSr can become hands free. See the picture on

Stonefisks blog. I presume that this would make the use legal???

 

I haven't managed to get this to work for me yet as I can't find double sided sticky tape that will hold it.

 

Appologies if this link doesn't work but its the first time I've tried to insert one - in any case you want his October 31st blog entry.

[+Billy Twigger]

Really, Officer...if you would only take the quickest look at my GPSr I think you'll find.........

 

 

P.S. Nice to see Alex McLeish has a new job?

Edited November 7, 2004 by Billy Twigger

[+Naefearjustbeer]

Really, Officer...if you would only take the quickest look at my GPSr I think you'll find.........

 

 

P.S. Nice to see Alex McLeish has a new job?

that you are correct you were not speeding on this section of road but can you explain this maximum speed displayed???

[+stu_and_sarah]

that you are correct you were not speeding on this section of road but can you explain this maximum speed displayed???

Well, mine says 527mph. I hope they don't think our cachemobile is capable of that.

 

Stu

[+The Hokesters]

If you have climatic control on a VW here is a different aproach. I have tried it and using the approach at the end of the description my Golf GDi worked great.

This didn't work on our Audi A2

[+Gary & Jane]

The recently introduced law which prohibits the use of a hand-held mobile phone also covers hand-held GPS units.

This page on the Department for Transport website states that:

 

"Use of devices other than mobile phones are only prohibited if the device performs an interactive communication function by sending and receiving data. If the device does not perform this type of function, you can use the device without breaching the regulations."

 

As a GPS receiver doesn't transmit data you shouldn't be breaking the law (as long as you operate it 'safely' of course).

 

Gary

Edited November 8, 2004 by Gary & Jane

[+kbootb]

The recently introduced law which prohibits the use of a hand-held mobile phone also covers hand-held GPS units.

This page on the Department for Transport website states that:

 

"Use of devices other than mobile phones are only prohibited if the device performs an interactive communication function by sending and receiving data. If the device does not perform this type of function, you can use the device without breaching the regulations."

 

As a GPS receiver doesn't transmit data you shouldn't be breaking the law (as long as you operate it 'safely' of course).

 

Gary

They might not be able to get you with the mobile phone law, but you can be done for eating a kit-kat or drinking from a bottle, so the use of a GPS would fall into the same category.

 

Driving without due care and attention I think would cover it.

[+The Hokesters]

If you ever get accused of driving without care and attention then just say that there was a wasp in the cabin and you are ver very scared of wasps!

 

Not sure if this will get you off the hook if you are having a shave, putting on your lippy, eating a sandwich and chugging a cold beer though