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GPS speed accuracy Vs speedometer?
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Badgerman26
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PostPosted: Mon Jan 14, 2008 6:39 pm    Post subject: GPS speed accuracy Vs speedometer? Reply with quote

My TTGO520 speed is regularly up to 6mph slower than on the speedometer (depending on the speed). Which is most accurate?
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tgold
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PostPosted: Mon Jan 14, 2008 6:48 pm    Post subject: Re: GPS speed accuracy Vs speedometer? Reply with quote

Badgerman26 wrote:
My TTGO520 speed is regularly up to 6mph slower than on the speedometer (depending on the speed). Which is most accurate?
Your GPS receiver is the accurate one.
Tony
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if1977
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PostPosted: Mon Jan 14, 2008 6:52 pm    Post subject: Reply with quote

I am not really sure if i agree with you tgold. This topic has been spoke about recently(not sure where) and it is stated that you should not rely on the TT speed as it can be thrown off by inclines and declines etc. Always go on your speedo and if i remember right there is a 5% + 2MPH toleration for the speedo also so if you stick to what the speedo says (Not the satnav) you should be ok and within the speed limit.
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dhn
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PostPosted: Mon Jan 14, 2008 7:04 pm    Post subject: Reply with quote

if1977 wrote:
I am not really sure if i agree with you tgold. This topic has been spoke about recently(not sure where) and it is stated that you should not rely on the TT speed as it can be thrown off by inclines and declines etc. Always go on your speedo and if i remember right there is a 5% + 2MPH toleration for the speedo also so if you stick to what the speedo says (Not the satnav) you should be ok and within the speed limit.


I, too, was--am--of the belief that the satellites provide more accurate speed readings than the speedometer. While your point is well taken regarding inclines, etc., also remember that your speedometer reading will be thrown off if you have a tire size installed that is NOT specifically designed for your car.
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Andy_P
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PostPosted: Mon Jan 14, 2008 7:14 pm    Post subject: Reply with quote

Every post on this subject (and there's lots!) conclude that the SatNav speed display is remarkably accurate and nearly all car speedos over-read by 5-10% when they come out of the factory.

The most recent discussion also debates the point about inclines and curves, and we reckon the errors involved are pretty insignificant.
But to be safe, you could always just look at the speed readings when on the straight and level and see what your speedo reads, then use that spot on the dial as your maximum.
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tgold
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PostPosted: Mon Jan 14, 2008 7:16 pm    Post subject: Reply with quote

if1977 wrote:
I am not really sure if i agree with you tgold. This topic has been spoke about recently(not sure where) and it is stated that you should not rely on the TT speed as it can be thrown off by inclines and declines etc. Always go on your speedo and if i remember right there is a 5% + 2MPH toleration for the speedo also so if you stick to what the speedo says (Not the satnav) you should be ok and within the speed limit.
Yes, AFAIK all consumer GPS receivers do report 2D "velocity" so any incline will affect the reading. But in general a GPS receiver will indicate within 0.5mph of true while the vehicle manufacturer's indicator is deliberately made to to read too high in order to comply with the construction regulations that require a speedometer never ever to read less than the true road speed.
Tony
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if1977
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PostPosted: Mon Jan 14, 2008 7:32 pm    Post subject: Reply with quote

I think that we are going over the thread again but what i am going to do is when i go to work tomorrow ( I am a vehicle technician for a franchise dealer but cannot say which type of car) i am going to carry out a PDI (Pre-Delivery-Inspection which is to check that brand new cars are safe for the road) and while on the road test i will connect the diagnostic machine to get a perameters reading for that vehicle which has no adjustments for the speedo and is the true reading of the car with standard and correct tyres and pressures for that model of car and i will also have my TT running alongside and will record the speed difference between them both and will post my findings tomorrow. I will do it at a steady 30mph on a straight road and also at 70 mph on a straight strech of the motorway. Any other ideas for testing please post them as long as they are safe to do and within the speed limits. Wink
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tgold
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PostPosted: Mon Jan 14, 2008 7:41 pm    Post subject: Reply with quote

if1977 wrote:
I think that we are going over the thread again but what i am going to do is when i go to work tomorrow ( I am a vehicle technician for a franchise dealer but cannot say which type of car) i am going to carry out a PDI (Pre-Delivery-Inspection which is to check that brand new cars are safe for the road) and while on the road test i will connect the diagnostic machine to get a perameters reading for that vehicle which has no adjustments for the speedo and is the true reading of the car with standard and correct tyres and pressures for that model of car and i will also have my TT running alongside and will record the speed difference between them both and will post my findings tomorrow. I will do it at a steady 30mph on a straight road and also at 70 mph on a straight strech of the motorway. Any other ideas for testing please post them as long as they are safe to do and within the speed limits. Wink
I am surprised that a vehicle technician believes inflation pressure affects the rolling circumference of a tyre or the accuracy of the speedometer. Anyway, I look forward to reading your results.
Tony
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if1977
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PostPosted: Mon Jan 14, 2008 7:45 pm    Post subject: Reply with quote

I was meaning that there would be no reason for anybody to say that it could cause a problem to the test and also when new cars come from the factory the tyre pressures are about 50-60 psi and that could be quite dangerous on the road.
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jmk
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PostPosted: Mon Jan 14, 2008 8:07 pm    Post subject: Reply with quote

tgold wrote:
I am surprised that a vehicle technician believes inflation pressure affects the rolling circumference of a tyre or the accuracy of the speedometer.

If you over inflate a tyre it wears the middle of the tread out... Is this because the radius has increased in the middle? - I think so...
John.
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tgold
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PostPosted: Mon Jan 14, 2008 8:33 pm    Post subject: Reply with quote

jmk wrote:
tgold wrote:
I am surprised that a vehicle technician believes inflation pressure affects the rolling circumference of a tyre or the accuracy of the speedometer.

If you over inflate a tyre it wears the middle of the tread out... Is this because the radius has increased in the middle? - I think so...
John.
The extra wear is caused by the ballooning side walls pulling the sides of the tread away from the road so that the centre bears most of the load, but I believe no practical amount of over inflation can stretch the tread belting to any measurable degree.
Tony
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jmk
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PostPosted: Mon Jan 14, 2008 10:02 pm    Post subject: Reply with quote

tgold wrote:
The extra wear is caused by the ballooning side walls pulling the sides of the tread away from the road so that the centre bears most of the load, but I believe no practical amount of over inflation can stretch the tread belting to any measurable degree.
Tony

The way I see it is that the over inflation reduces the contact area of the tyre with the road surface. This lifts the wheel up and increases the height of the centre of the wheel above the road surface. This increases the radius and it follows on that the rolling circumference also increases - hence the speed change.
BUT, - I have been known to be wrong before... Wink
John.
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andy-c1
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PostPosted: Mon Jan 14, 2008 11:28 pm    Post subject: Reply with quote

well i'm about to find out which is true as i passed a camera car the other night and when i was passing he reached into the van ,which doesn't bode well for me ,as i do drive within the limit of tomtom if the siren goes off i do slow down a bit till the reading goes white again
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jspencer
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PostPosted: Tue Jan 15, 2008 4:20 am    Post subject: Reply with quote

if1977 wrote:
I am not really sure if i agree with you tgold. This topic has been spoke about recently(not sure where) and it is stated that you should not rely on the TT speed as it can be thrown off by inclines and declines etc.

Not enough to make a difference. One of the highest road grades is near the top of Mt. Washington, where it's 22%. That's equal to an angle of about 12.4°, which translates into a speed error (if you just measure horizontal speed) of about 2%. So if you are moving 50 mph on a 22% grade, your GPS will measure it at about 49 mph.

But 22% is way off the chart. The road up Mt. Washington averages 12% and most normal road grades are considerably less than that. At 12% you will see about a third of a mph error at 50 mph. I can't read my speedometer closer than about 2 mph with any confidence, so a third of a mph is definitely in the noise.

Jon
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mikealder
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PostPosted: Tue Jan 15, 2008 9:09 am    Post subject: Reply with quote

I wonder if this thread will go the same wandering path the last one did that related to speedo accuracy, a quick summary:

mikealder wrote:
Lets take a quick look at the previous three pages, this thread has kept me amused over the last few days, it starts with a simple and often asked question:

teamgers wrote:
Hi, can anyone tell me which is the more accurate, the speed indicated on your sat nav? or, the speed indicated on your speedo in the car?


Some contradictory answers before the question was answered:

Anita wrote:
PaulB2005 wrote:
The Sat Nav will be more accurate as speedos tend to under read the speed.

I think you'll find speedos tend to over read, i.e. the speed shown is faster than the actual speed, in my Ka by about 10%.


Some mathematics (part one):

Andy_P2002 wrote:
Reap wrote:
going up or down hill will make it read slower than it is.


Quick bit of Pythagoras...

On a 1 in 10 hill, for a horizontal travel of 100yds that the GPS registers, you actually travel 100.04998750624609648232582877001 yds - meaning an error of -0.05% on the speed indicated. Hardly significant!


Some redefinition of the shape of a triangle:

philpugh wrote:
The 'base' of your triangle needs to be curved to match the spheroid of the datum (eg WGS84). So the amount of under-reading will depend upon if you are going N-S or E-W (and directions in between) and where on the surface of the earth you are travelling.


Some more Mathematics, and proof that slide rules can still beat the calculator:

Andy_P2002 wrote:
snudge wrote:
I had a new slide-rule for Xmas and I can't get the figure that Andy does?


Also very possible!

I think I added a nought that I shouldn't have...
I did it for a 10x1 triangle which gives 10.04987562112089027021926491276 and scaled up (badly!)

How about 100.4987562112089027021926491276?


Insomnia starts to kick in:

Oldboy wrote:
I won't sleep tonight now ...... Surprised


M8TJT wrote:
Jeez, I'm loosing sleep over this as well.


The evils of drink take over:

Andy_P2002 wrote:
I'm off to the pub.... Guinness


M8TJT wrote:
Yo, Dude. Good idea. I wish that I had thought of that before posting my last.


How the heck does a Snooker Table fit in to this lot?

DennisN wrote:
Andy_P2002 wrote:
I remember at school a long time ago, having to work out the curvature required on a snooker table to stop all the balls running into the middle! Laughing

Off Topic When I were a lad, snooker tables hadn't been invented - I suppose some crafty would-be snooker table manufacturer had all you boys working at this because it saved him paying somebody to work it out.


Discussions about railways:
DennisN wrote:
A far more accurate method was used when I were a lad. You counted the number of clicks as you went over the joins in the rails, then divided them by something and it told you how fast you were going. (You sort of said "Diddly dee, diddly dah, diddly dee, diddly dah and your pal counted them).


Rail Length:

xda wrote:
Doesn't work any more Dennis, they have done away with the old standard length of rail joined by fish plates, which gave you the expansion gap every 100 ft, which gave you the Diddly Dee, Diddly Dahs. You have to find other games to keep the kids busy now.


Counting objects (we are back to combating insomnia)

DennisN wrote:
There are still two alternatives ..
1. Count telegraph poles.
2. Count sheep.


Oldboy wrote:
DennisN wrote:
2. Count sheep.
Used to be cows when going past Milton Keynes. Laughing


Now I was going to ask if we can keep this “On Topic” or should it be moved to the “Playground” – But I see Phil has pulled it back on track for now - Mike


The post can be found Here - Mike
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