How accurate is the SV speedo?

[skithepowder]

Just wondered if anyone knows how accurate the SV speedo is? Maybe had GPS of some sort that is more accurate to compare it to?

I know that in my car, my speedo overreads by about 10%, is it a similar case on the SV? Is there any difference between pointy and curvy?

[Bluefish]

Just wondered if anyone knows how accurate the SV speedo is? Maybe had GPS of some sort that is more accurate to compare it to?

I know that in my car, my speedo overreads by about 10%, is it a similar case on the SV?

yep, they all over read, a policeman/woman will let you know if your going too fast, ;)

[sath182]

Confirmed with GPS that my '09 is 9% fast while on a trip around the States. Breakes it in to factors of eleven; 55=50, 77=70, etc. Don't know about the curvy though.

[kaivalagi]

I can confirm also from my GPS on the pointy, 77 actually 70, 66 actually 60 etc...I'd hazard a guess the curvy is the same

[andrewsmith]

rule of thumb for the overreads is 10%
There are some hondas and Yams that are 5%

My curvy seems to be Italian in this area, anything between 1% and 10%, but it was best part cured after a suspension refresh

[rictus01]

have a look at some of the top speed claims in one of the go faster threads, it'll give you the answer you seek......;)

[keith_d]

IIRC UK regulations require a speedometer to read between 0% and 10% above actual speed. Car manufacturers aim for about 5% over to allow for things like variation in tyre size.

I've not tested my bikes, but I wouldn't be surprised if they were somewhere near the 10% mark. Bike manufacturers have to allow for the fact that we'll fit all kinds of rubber to our bikes, so I'd expect plenty of margin to avoid under-reading.

Keith.

[Bluefish]

so does this mean as your tyre wears down your speedo gets more accurate? edit this would depend on where the reading was taken from, ie if of the drive then it wouldn't make any differance?.

[andrewsmith]

I'm sure over read it based on the Homoglated tire (in the curvy's case MEZ4) in run-in condition

[kaivalagi]

so does this mean as your tyre wears down your speedo gets more accurate? edit this would depend on where the reading was taken from, ie if of the drive then it wouldn't make any differance?.

Whatever the little difference tyre sizing and wear will make it will make it no matter how the speedo readings are taken...whether from the wheel or sprocket rotation if the wheel diameter reduces you'll get marginally less distance and therefore less speed for the same rotation if that makes sense (and vice versa, if the tyre is a bigger diameter then the speed achieved is greater for the same rotation).

I'd be interested to know how much of an actual difference it would make though...I am thinking it's probably not worth even considering unlike the deliberate massive over read on the speedo itself...10% on the speedo compared to maybe 1% at most for tyre variations most likely...

edit: As a tyre wears the speed will reduce and therefore the reading will seem even more exaggerated, so less accurate....(I think...)

[-Ralph-]

You can measure the diameter of your wheel and tyre, you know how many mm of tread you had new, you know how many mm of tread are left. You can easily calculate a circumference from a diameter, and convert that into a distance travelled and figure out a number of rotations for a given distance. do it for a knackered tyre and a new tyre and you can easily calculate the difference in speed reading. Personally I can't be arsed.

My vehicles

Pointy SV + 10%
Yamaha XT600 -3% (yes it under reads!)
Seat Ibiza + 10%
Vauxhal Vectra + 5%

[kaivalagi]

Personally I can't be arsed.

Me neither that's why I was hoping someone had a idea of the variation....life's too short though :)

[Specialone]

I know for a fact ALL VDO manufactured clocks are factory made with an over read of 5%.
My pointy also over read exactly the same as others have mentioned.

[musne]

So the question is:

What is the best device to get to enable you to have the most/more accurate reading of what speed you're travelling at?

GPS?

[kaivalagi]

GPS helps you understand but you don't need it after a while anyway

You could go out and buy a Speedohealer? or whatever the equivalent are...
Manufacturer's site here: http://www.healtech-electronics.com/

Or just get used to it and know that 77=70, 66=60, 55=50 :)

Borrow a gps for a ride to check what your bike comes out then you're done

[sath182]

If you go the route of a speedohealer, keep in mind that the Odometer used the speed sensor as well so your millage will end up 9% short of actual.

[kaivalagi]

If you go the route of a speedohealer, keep in mind that the Odometer used the speed sensor as well so your millage will end up 9% short of actual.

Mmmmmm, here's a question...Is the odometer accurate or does it also have 9% more than actually ridden as standard?

[-Ralph-]

Mmmmmm, here's a question...Is the odometer accurate or does it also have 9% more than actually ridden as standard?

9% more.

I think musne was asking if GPS is accurate, the answer is, for the purposes of measuring vehicle speed to within 1-2 mph, yes it is.

[sath182]

From what I understand, the Odo is mostly accurate. Just the speedo has the over-read. I did a search on google for "speedohealer odometer" and the first few links have statements that follow this line of thought, but I can't find anything hard from Healtech. I was looking into one of these a while back but remember reading that it would make the Odo wrong so I let it go. I like not running out of gas :P

[kaivalagi]

Good to know, the newer dashs are digital rather than the old fashioned wind up things so it would make sense they are accurate in distance and inflated in speed

Personally I think just knowing the differences in actual speed versus what the dash is telling you is enough anyway

[mikerj]

Good to know, the newer dashs are digital rather than the old fashioned wind up things so it would make sense they are accurate in distance and inflated in speed


It doesn't make any difference, there is no inherent reason why a digital odometer would be more or less accurate than a mechanical one. A mechanical speedometer is different as it relies on the strength of magnets and a hair spring and has precision bearings that get gunked up over time, so accuracy is inevitably compromised, but an odometer is a simple geared system with no opportunity for the calibration to move other than through normal tyre wear/inflation or failure of some internal part.

[kaivalagi]

I am not sure you got where I was coming from, based on earlier posts?

Firstly can we assume a new type dash which has a digital speedo doesn't compromise the digital odometer even though it is 9% more than actual speed...i.e. if it reads 77mph even when doing 70 will the odometer still track correctly and record 70 miles for an hours travel or will that also be inflated by that 9/10% margin too?

I was suggesting if the odometer was accurate on a new dash even with this inflation would the older type still suffer as the odometer was driven by the speed recorded?

Please enlighten me though as I am only making assumptions....would the odometer be inflated by 10% just like the speedo regardless of tech or not?

[sath182]

Time for a road trip. Someone? Anyone? I can't do it, they wouldn't let me bring mine to Afghan land.

[squirrel_hunter]

Confirmed with GPS that my '09 is 9% fast while on a trip around the States.

I can confirm also from my GPS on the pointy, 77 actually 70, 66 actually 60 etc...

What makes you so sure your GPS is accurate?

[davepreston]

depends om tyre size does it mmmmmmmmmmmmmm
my thou runs off the gears :)

[kaivalagi]

What makes you so sure your GPS is accurate?

mmmm, because it's GPS, granted accuracy for position for civvy land is not the best but at speed it's plenty good enough...and if more than 1 of us are getting the same results then that's confirmation enough for me

[-Ralph-]

mmmm, because it's GPS, granted accuracy for position for civvy land is not the best but at speed it's plenty good enough...and if more than 1 of us are getting the same results then that's confirmation enough for me

+1, my V7 sat nav and my Samsung GPS smartphone and a colleagues HTC GPS smartphone agreed to one decimal point what the speed was on a motorway (though they quite possibly use the same GPS chipset), when using cruise control. It was a long journey, and we were bored!

Bluepete has said in the past that his TomTom is bang on compared to the calibrated speedos in his job cars.

We could get into a whole argument about how accurate they are for positioning or altitude (you only need to speak to someone who does geocaching to know that one can be 100 metres out from the reading of another, not that I understand why), but when it comes to measuring a constant speed, they are accurate enough for my needs.

[kaivalagi]

Next time out on the bike (and if I remember) I'm going to use my android phone to record my route using "My tracks" or similar so I can find out the distance travelled versus what the trip meter says...or use the tomtom if it has the feature (?). I am just not convinced either way if the odometer on the bike is accurate or inflated like the speedo....if inflated I'd say getting a speedohealer might be cost effective if you use your bike a lot!

[sath182]

Next time out on the bike (and if I remember) I'm going to use my android phone to record my route using "My tracks" or similar so I can find out the distance travelled versus what the trip meter says...or use the tomtom if it has the feature (?). I am just not convinced either way if the odometer on the bike is accurate or inflated like the speedo....if inflated I'd say getting a speedohealer might be cost effective if you use your bike a lot!

+1 I like this idea. Hadn't really thought of it. Just remember to reset your trip ;)

[squirrel_hunter]

mmmm, because it's GPS

And you know thats accurate because?

granted accuracy for position for civvy land is not the best but at speed it's plenty good enough...

Now I know that position for civilian applications is not as accurate as it could be (based on first hand system developer accounts), so why would the speed be?

and if more than 1 of us are getting the same results then that's confirmation enough for me

And may I present to you:

Confirmed with GPS that my '09 is 9% fast while on a trip around the States. Breakes it in to factors of eleven; 55=50, 77=70, etc.

I can confirm also from my GPS on the pointy, 77 actually 70, 66 actually 60 etc...

I'm just wondering you see if by your reasoning more than one has the same results, how do you know which one is accurate, the GPS or the speedo? And based on what?

[mikerj]

And you know thats accurate because?

Now I know that position for civilian applications is not as accurate as it could be (based on first hand system developer accounts), so why would the speed be?


Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

A GPS system will be more accurate than all but the most carefully calibrated speedometers under the majority of conditions. Even consumer grade devices have a typical accuracy of 0.1mph, though under some conditions that can degrade a little, maybe +-0.5mph.

[timwilky]

depends om tyre size does it mmmmmmmmmmmmmm
my thou runs off the gears :)

Yeah, so what!

That just means the speedo is proportional to the final drive etc. So even worse accuracy than an 650 if you change your sprocket ratios. Tyre wear will still also be a minor factor.

[kaivalagi]

I'm just wondering you see if by your reasoning more than one has the same results, how do you know which one is accurate, the GPS or the speedo? And based on what?

Thought you might be interested in this from here (http://en.wikipedia.org/wiki/Speedometer#GPS):
"GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed."

And this for kicks: http://www.heraldsun.com.au/news/victoria/gps-beats-radar-gun/story-e6frf7kx-1111113864822

Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

A GPS system will be more accurate than all but the most carefully calibrated speedometers under the majority of conditions. Even consumer grade devices have a typical accuracy of 0.1mph, though under some conditions that can degrade a little, maybe +-0.5mph.

Thanks, sometimes I know but forget why I do, not enough room up there sometimes for the details :)

[squirrel_hunter]

Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate? Further Doppler Shift as I understand it is the same theory that is used in radar guns as issued to the police to calculate speed, however these devices have been proved to be inaccurate, due in part to target slippage, something that a moving device could suffer from...

Thought you might be interested in this from here (http://en.wikipedia.org/wiki/Speedometer#GPS):
"GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time.

As I said above the calculation is still subject to position which is a known inaccurate. Though the points on wheel size and drive ratios are good if the parts have been changed from the original.


And this for kicks: http://www.heraldsun.com.au/news/victoria/gps-beats-radar-gun/story-e6frf7kx-1111113864822


Yes but they say that the ticket should not have been revoked.

[mikerj]

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate?


Doppler is the primary mechanism for speed calculation, I honestly can't remember why or when a contribution of trackpoints is added. A doppler based system can have exceptional accuracy (http://www.google.co.uk/url?sa=t&source=web&cd=3&ved=0CDAQFjAC&url=http%3A%2F%2Fnujournal.net%2FHighAccuracySpeed .pdf&ei=nDF5TcLQCsKqhAfv7Iz3Bg&usg=AFQjCNHG8QwHEbk3TagPM6AA5Fpz2KZ4ig&sig2=ZUBHBpy2lbziUTVBVRmuxA)



Further Doppler Shift as I understand it is the same theory that is used in radar guns as issued to the police to calculate speed, however these devices have been proved to be inaccurate, due in part to target slippage, something that a moving device could suffer from...


Are you maybe thinking about laser speed guns? They can suffer quite badly from slip error due to the relatively narrow divergence of the beam and the range over which they can be effective. Radar guns have less of a problem as the beam diverges much more, but it can still happen. GPS simply isn't affected by this since a reflected signal isn't used at all, the GPS receiver directly measures the relative shifts in the satellite carrier frequencies.

[kaivalagi]

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate?

I'm happy with this principle of the speed being much more accurate regardless of positional error, but I went looking for proof. Here's more on it: http://eprints.ucl.ac.uk/146583/

From the text:
"The speed determined by the GPS receiver was within 0.2 ms(-1) of the true speed measured for 45% of the values with a further 19% lying within 0.4 ms(-1) (n = 5060). The accuracy of speed determination was preserved even when the positional data were degraded due to poor satellite number or geometry. GPS data loggers are therefore accurate for the determination of speed over-ground in biomechanical and energetic studies performed on relatively straight courses. Errors increase on circular paths, especially those with small radii of curvature, due to a tendency to underestimate speed."

0.5 metres per second being an absolute worst case scenario is a smidgen over 1 mph, so worst case when speed is measured on an oval track like in the test (not the best accuracy to begin with as mentioned) we are looking at 0.5 mph either way...

If you still don't buy into the concept I am not sure what else I can say to convince you....all I can say is read about some of the science behind it

edit: what he ^^^ says also :)

[-Ralph-]

A few quotes from websites saying they are accurate, a university study on it suggesting the same, quite a few people whose experience suggests their various GPS devices all agree, both in this thread and in others on the subject.

SH, if you think they are inaccurate at measuring a constant speed in a straight line which is what we are talking about here (ie: 70mph = 77), to more than 1 mph, then instead of asking why we think the sat nav IS accurate, then questioning the replies, why not produce some evidence as to why you think they are NOT?

[danf1234]

Not very according to my Sat Nav.

[squirrel_hunter]

Doppler is the primary mechanism for speed calculation, I honestly can't remember why or when a contribution of trackpoints is added. A doppler based system can have exceptional accuracy (http://www.google.co.uk/url?sa=t&source=web&cd=3&ved=0CDAQFjAC&url=http%3A%2F%2Fnujournal.net%2FHighAccuracySpeed .pdf&ei=nDF5TcLQCsKqhAfv7Iz3Bg&usg=AFQjCNHG8QwHEbk3TagPM6AA5Fpz2KZ4ig&sig2=ZUBHBpy2lbziUTVBVRmuxA)

Thats a fantastic paper, thanks.

Are you maybe thinking about laser speed guns? They can suffer quite badly from slip error due to the relatively narrow divergence of the beam and the range over which they can be effective. Radar guns have less of a problem as the beam diverges much more, but it can still happen. GPS simply isn't affected by this since a reflected signal isn't used at all, the GPS receiver directly measures the relative shifts in the satellite carrier frequencies.

Yep was thinking more about laser than radar. But its interesting to know that GPS isn't effected in the same way.

Here's more on it: http://eprints.ucl.ac.uk/146583/

Cheers for that artical as well.

[squirrel_hunter]

A few quotes from websites saying they are accurate, a university study on it suggesting the same, quite a few people whose experience suggests their various GPS devices all agree, both in this thread and in others on the subject.

SH, if you think they are inaccurate at measuring a constant speed in a straight line which is what we are talking about here (ie: 70mph = 77), to more than 1 mph, then instead of asking why we think the sat nav IS accurate, then questioning the replies, why not produce some evidence as to why you think they are NOT?

I've never said that GPS is inaccurate, nor do I think its inaccurate. I have simply been wondering how it is so accurate compared to a speedo. I have previously read that GPS is more accurate than a speedo, but no one has provided the answer as to why. So when it popped up again I asked and kaivalagi and mikerj have been kind enough to provide me with an answer, thanks guys.

[kaivalagi]

I think the original issue of speedo accuracy was more centered around the deliberate adjust up in speed of the readings as imposed (indirectly) by law, which there is general agreement on. I think we got a little sidetracked with details :)

BTW I still need to find out if distance recorded by the odometer is 9/10% higher than actual just like the speedo...if it is that's just plain wrong... different bikes with varying accuracies would record varying falsely inflated mileage...a speedohealer at £60(?) might be worth it if that's the case...

[sath182]

I've never said that GPS is inaccurate, nor do I think its inaccurate. I have simply been wondering how it is so accurate compared to a speedo. I have previously read that GPS is more accurate than a speedo, but no one has provided the answer as to why. So when it popped up again I asked and kaivalagi and mikerj have been kind enough to provide me with an answer, thanks guys.

For the speed calculation in a satellite based GPS system there are two sets of measurements taken, Frequency Difference of Arrival (FDOA/Dopplar Shift) and Time Difference of Arrival (TDOA). Frequencies shift and become longer as they get further from the source. The same signal travels from your GPS (civilian GPS is two way) to multiple satellites. The satellites pick it up measure the shift in frequency and compares the time of arrival with that of other satellites. That's where the geo-location comes from. Now for speed. the updates are usually going to be on a set timer, say every 3 seconds. If your position changed x amount in 3 seconds, you are going **mph. Add up the most recent 20 or so of those updates and you get an average speed. Yes, the location may be off, but software has programing in it to exclude locational data that doesn't meet it's confidence requirements.


BTW I still need to find out if distance recorded by the odometer is 9/10% higher than actual just like the speedo...if it is that's just plain wrong... different bikes with varying accuracies would record varying falsely inflated mileage...a speedohealer at £60(?) might be worth it if that's the case...

I'm a little blurry about the details, but I was working out what my Odo should say for different checkpoints on my trip and it seemed pretty close. I would like to see a larger sample size than 30-50miles though.

[-Ralph-]

I've never said that GPS is inaccurate, nor do I think its inaccurate. when it popped up again I asked

If you still don't buy into the concept I am not sure what else I can say to convince you....

Fairy nuff, it just looked a lot like you were in dissent with the concept.

[squirrel_hunter]

I think the original issue of speedo accuracy was more centered around the deliberate adjust up in speed of the readings as imposed (indirectly) by law, which there is general agreement on. I think we got a little sidetracked with details :)

True.

BTW I still need to find out if distance recorded by the odometer is 9/10% higher than actual just like the speedo...if it is that's just plain wrong... different bikes with varying accuracies would record varying falsely inflated mileage...

Now thats an interesting point.

[kaivalagi]

I'm a little blurry about the details, but I was working out what my Odo should say for different checkpoints on my trip and it seemed pretty close. I would like to see a larger sample size than 30-50miles though.

Thats interesting, from a design perspective I would expect the speed inflation to only be on the screen and not to affect the odometer too so hopefully that is the case.

Today looks to be a nice day, if I finish work early enough I'll go on a little bimble for half an hour with my tracks running and hopefully have something to go on....even if not very accurate we should see a distance at one end or the other of the accuracy scale :thumbsup:

[kaivalagi]

I went out for a ride and although overcast and not great for my crappy gps on my phone I got the results as follows:

Odometer: 37.8 miles
GPS on Android via My Tracks: 36.83 miles

That's around 2% difference which makes me believe that the odometer is not inflated by 9/10% like the speedo is....Also the GPS is most likely reading low because of lost signals around bends etc that would give straight line distances where this is not the case...see the image below, where there is no blue the signal was lost...

http://i781.photobucket.com/albums/yy91/kaivalagi/Scratch/mytrack.jpg

I will give this another go when the weather is better, like I say it was overcast today, I nearly didn't bother as I didn't get a gps fix from home, I tried again once I'd travelled for a bit



Anyway I think this raises another question, for all those that have fitted speedohealers are the odometer readings on their bikes lower than they should be because of the reduced speeds on the clock !!!??? :) That's for someone else to figure out, I'm never going to bother fitting one.....

[sath182]

I went out for a ride and although overcast and not great for my crappy gps on my phone I got the results as follows:

Odometer: 37.8 miles
GPS on Android via My Tracks: 36.83 miles

That's around 2% difference which makes me believe that the odometer is not inflated by 9/10% like the speedo is....Also the GPS is most likely reading low because of lost signals around bends etc that would give straight line distances where this is not the case...see the image below, where there is no blue the signal was lost...

http://i781.photobucket.com/albums/yy91/kaivalagi/Scratch/mytrack.jpg

I will give this another go when the weather is better, like I say it was overcast today, I nearly didn't bother as I didn't get a gps fix from home, I tried again once I'd travelled for a bit



Anyway I think this raises another question, for all those that have fitted speedohealers are the odometer readings on their bikes lower than they should be because of the reduced speeds on the clock !!!??? :) That's for someone else to figure out, I'm never going to bother fitting one.....


Sounds like a good next step to me. Another one to try and increase the accuracy might be a long straight line and back. That should avoid some of the signal loss errors.

[-Ralph-]

I'm not sure if an inaccurate odometer would matter anyway. All of the odo's on all of the bikes would be out by approximately the same amount if that were the case. Even if it one odo was accurate, and another was 10% out, you are not going to look at two bikes in autotrader, one with 30,000 and one with 33,000, and choose between them based upon mileage. You could say you would save 10% on your servicing costs, but even at main dealer prices, that's £20 every 4000 miles, and it's not a bad thing if your bike gets serviced more regularly than the schedule anyway.

[kaivalagi]

Very true, I was getting carried away with the detail, I now know that the odometer is accurate where as the speedo id inflated...that's a nagging question put to bed anyway :)

Atleast if there is a discrepancy it's the right side of actual to help not hinder when it comes to servicing...