Which is more accurate....?

[woody67]

........ the digital speedo in my Honda Civic, or my Garmin Sat Nav which also displays m.p.h? Reason I ask, there is a 4-6 m.p.h difference between the two - the sat nav showing slower, BUT the sat nav reading "feels" the more accurate! :shock:

Technically I could set the sat nav to display a maximum speed in a single journey, so if it showed 39.7 m.p.h as max speed and I got done for doing over 40 m.p.h in a 40 zone courtesy of the Boys in Blue, would my recorded data stand up in court? :shock:

Bet somebody knows :wink:

Thanks all

Mark

 

[christoph clark]

Found this which is what I thought.

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Modern speedometers are said to be accurate within 5% but as this is legislated accuracy (ISO 1997), this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed. Although levels of some 3 km/h, or 3% are also used, where tough enforcement is used. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[1] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the

percent error = 100x("standard diameter"/"new diameter" - 1).

GPS devices may indicate the true speed of travel on the user interface. Unlike instrumental speedometers which provide a continious reading, the GPS speed readouts have a one second update interval.

The reading is based on reception of data from the satellites in orbit, and is therefore independent of the car's transmission components. Discrepancies between the two readings may be caused by instrument error (on the vehicle), or by changing directly influencial factors, such as tire sizes.

and this

Car speedometers use a sensor on the driveshaft to measure your speed as a function of wheel revolution. Because the tires are made out of rubber, their diameter varies with speed, load, and temperature, even when inflated to their recommended pressure. You'll never get an accurate reading from a car speedometer; if you have ever seen a test vehicle on a track, you might have seen something like this trailing it:
It's called a fifth wheel speedometer, and it was designed to compensate for the inaccuracies I mentioned.

 

[Steve Maskery]

I'd say the SatNav is likely to be more accurate.

Car speedo's, by law, are allowed to show over the actual by up to 10% but not under at all. To make measuring instruments accurate costs money, and the greater the accuracy, the higher the cost. So manufacturers can build a basic instrument that falls within the rather generous margin of error without spending too much money.

My car reads about 10% over, as measured by those LED reminders on poles. If I'm doing 30 the road sign tells me I'm doing 27 or 28.

S
Edit - Beaten!

 

[MikeG.]

This will get you thinking.......

........it doesn't matter how inflated or flat your tyre is, one revolution will cover the same distance. Tyre pressure therefore has no effect on the speedo reading.


I concur with everything else in Christoph's post though.

I'll just go and find the book with that little gem in it.....

Mike

 

[christoph clark]

it doesn't matter how inflated or flat your tyre is, one revolution will cover the same distance. Tyre pressure therefore has no effect on the speedo reading.

Not so sure.
If your tyre was flat, for example, the diameter would be less then fully inflated consequently it would travel less in distance per revolution.

 

[CHJ]

........it doesn't matter how inflated or flat your tyre is, one revolution will cover the same distance. Tyre pressure therefore has no effect on the speedo reading.

I presume it's because the tyre periphery remains the same length, give or take an odd mm. It's just behaves much like a caterpillar track if deflated and as long as it does not slip on the rim the axle has to traverse the same distance.

 

[cutting42]

........it doesn't matter how inflated or flat your tyre is, one revolution will cover the same distance. Tyre pressure therefore has no effect on the speedo reading.

I presume it's because the tyre periphery remains the same length, give or take an odd mm. It's just behaves much like a caterpillar track if deflated and as long as it does not slip on the rim the axle has to traverse the same distance.

OK Then, how does my BMW run flat tyre sensor detect a puncture then as it has no pressure sensor in the wheel. The manual states it monitors the wheel speed compared with the other wheels.

 

[christoph clark]

how does my BMW run flat tyre sensor detect a puncture then as it has no pressure sensor in the wheel

Pressure monitoring system depends on the year BMW . Used to work by comparing the wheel rotational speed as measured by the ABS wheel sensors. Low air pressure results in a deformed tire which results in a different circumferential length which results in a different rotational speed. After that they switched to a pressure monitor in each wheel which actually measures pressure and not rotational speed variations

 

[cutting42]

how does my BMW run flat tyre sensor detect a puncture then as it has no pressure sensor in the wheel

Pressure monitoring system depends on the year BMW . Used to work by comparing the wheel rotational speed as measured by the ABS wheel sensors. Low air pressure results in a deformed tire which results in a different circumferential length which results in a different rotational speed. After that they switched to a pressure monitor in each wheel which actually measures pressure and not rotational speed variations

Oooo, you sound like you know what you are talking about. I had a 55 plate 3 series that never gave me a single puncture warning in the three years I had it. I currently have a 58 plate 3 series that has had 2 fake pressure warnings and 1 real one and my wife has a 56 plate 3 series coupe with no messages either but she has only had it 4 weeks (new toy and my excuse for buying tools )

 

[Shultzy]

This will get you thinking.......

........it doesn't matter how inflated or flat your tyre is, one revolution will cover the same distance. Tyre pressure therefore has no effect on the speedo reading. Mike

I have to disagree. As the tyre is pumped up the contact patch gets thinner, the distance between the road and the centre of the axle increases, and the car travels further for one revolution. Therefore the converse happens as the pressure is reduced.

 

[StevieB]

I can confirm tyre diameter DOES affect speed and distance. When I biked to work (135 miles a day) I used to get a new front tyre approx every 12,000 miles. My journey length increased by approx 4 miles a day between a new tyre being fitted and an old one being removed simply because the tyre tread wore down and made the wheel circumference smaller.

As to whether tyre pressure actually decreases diameter however, I would think you would have to inflate or deflate by an awful lot to get the same degree of variation as tyre wear.

Just my twopenneth

Steve.

 

[cutting42]

When I biked to work (135 miles a day)

:shock: :shock: :shock: :shock: :shock:


edit - When I first read this I had a vision of a 6 hour cycle ride each day then I realised (hoped) your bike had a motor in it.

 

[Argee]

To answer the original question:

Technically I could set the sat nav to display a maximum speed in a single journey, so if it showed 39.7 m.p.h as max speed and I got done for doing over 40 m.p.h in a 40 zone courtesy of the Boys in Blue, would my recorded data stand up in court?

Short answer: No.

Longer answer: The Garmin will not have a certificate of calibration for use as a speed-checking device. Also, how would you prove (acceptably for a Magistrates' Court) that the time of your "recording" was during and included the time of the alleged offence?

The Garmin will be more accurate overall, providing it can find sufficient satellites to co-ordinate with. The more satellites it uses, the more accurate it will become , but it still has a speed-related lag over a speedometer, so I wouldn't use it as one, as it would be too distracting waiting for it to "settle."

Speed prosecutions are generally set (by ACPO) to kick in at 10% + 2mph over, in other words at 46 mph - but that's only a guide and zero-tolerance could be applied by an individual Force policy, I'd guess.

Use the speedo, it will be kinder to your wallet and licence in the long term.

Ray.

 

[newt]

I assume that the sat nav only gives correct speed when the car is moving on a horizontal plane.

 

[MikeG.]

Whilst that may be so, the steepest road in England is 1 in 4, and a quick bit of trig shows that the GPS error even there would only be 3%.....

Mike

 

[promhandicam]

tyre diameter will certainly alter the speedo reading as the circumference will change. this is why, as stevieb found his speedo reading varied depending on whether the tyre was new or old - the new tyre obviously having a larger diameter. A flat tyre will still have the same circumference as an inflated tyre so shouldn't make any difference to the speedo reading - particularly as the other 3 tyres will compensate.

Steve (another one!)

 

[MikeG.]

As the tyre is pumped up the contact patch gets thinner
Correct
, the distance between the road and the centre of the axle increases,
Correct
and the car travels further for one revolution
There is no logical link between the previous two assertions/ observations and this one. I suggest you take an old tyre, put a dress-makers tape-measure around it, then distort the tyre as much as you can, any way you can, and re-measure.

You'll surprise yourself!

.

 

[RobertMP]

Took a while for the penny to drop and agree with mike g.

I imagined a large cartwheel with spokes and some clever hub design that slides on the spokes and allows the axle to be at a well off centre position. One turn of the hub is one turn of the outside of the wheel whether the hub is central or off centre near the ground (as in a flat tyre) and will cover the same distance.

 

[Ironballs]

A side issue, but are there not some 1 in 3 roads in England?

 

[Shultzy]

As the tyre is pumped up the contact patch gets thinner
Correct
, the distance between the road and the centre of the axle increases,
Correct
and the car travels further for one revolution
There is no logical link between the previous two assertions/ observations and this one. I suggest you take an old tyre, put a dress-makers tape-measure around it, then distort the tyre as much as you can, any way you can, and re-measure.

You'll surprise yourself!

.

There is a logical link. Lets say at the correct tyre pressure the distance between the road and the centre of the axle is 12". Circumference is approx 72". Now blow the tyre up until the distance between the road and the centre of the axle is 18". Circumference is now approx 108". This can be done because the rubber expands and I believe its how top fuel dragsters increase their speed.

http://www.liveleak.com/view?i=be5_1216355697&c=1