Speedo Correction Is it possible

[dgp]

I think if it’s possible, hillbilly should go for it. Why not make the speedo as accurate as possible?
I will be interested if it can be done.

[Guest001]

It is quite common in the southern parts of Oz not to lock onto four satellites, we encountered this many times when data logging race cars and the Tiguan never tells you how many satellites it has access to where we could see it against the data. GPS signals available to non military users works to a tolerance which varies when it gets manipulated by the US authorities so when that happens the accuracy wanders around. We used physical sensors and overlaid the data with GPS and the GPS ALWAYS lagged in both speed and other readings without fail. I have connections with a family company which designs and builds this stuff and believe me it ain't fool proof until you get into the military grade GPS which they do.

Yeah well my wifes BF's son did most of the design work on the original Navmans in NZ . Sadly i wouldnt recommend one of them.

However they are accurate to a much closer distance than before the US military had them fudged up years ago and then they allowed it to be more accurate.. My Tomtom usually gets more than 4 (Most times) lol

[Ozsko]

Yeah well my wifes BF's son

A galaxy of relationships I don't want to even think about, is that like the bloke I know who's sons wife's brother can get me a deal on a car?. The US regularly changes the accuracy and you can see the GPS changing in a full day of data as it happens due to normal drift during the day. GPS is not the accurate thing that we all think it is, not by a long shot.

[Guest001]

, is that like the bloke I know who's sons wife's brother can get me a deal on a car?. .

No true story He did the original Navman and his wife who was also an engineer was one of the ones who developed the F& P dishdrawer

Quite a talented pair

I dont think we need to worry about all the nuances when just checking our cars speed really.

However I still would like to get mine a bit closer

[theresanothersteve]

Read this then and learn LOL

To calculate the Longitude, Latitude and Height position, a GPS receiver precisely measures the different speed of light (299,792 km/s) delays in the signals coming from 4 or more satellites. The distance to each satellite is calculated, and then using trilateration, the 3D position of the GPS antenna is calculated.


How GPS Receivers Work | HowStuffWorks

As far as how often it does it, its almost instant so as you slow your car so does the reading Its not like it updates ever 30 secs or something

The missing piece of the puzzle is bread crumbing. The GPS records your location as a 3d reference. Then it records the next. The distance between the points is calculated, along with the time elapsed between each point. This means when you are accelerating or decelerating the speed lags slightly behind the true speed.

At a constant, or near constant, speed the indicated speed is more reliable than that the speedo shows.

[Sydneykid]

Aside from GPS speed nothing will compensate for tyre wear affecting the distance travelled with each wheel rotation (ie; speedos and ODB outputs are simple rpm counters). Even brand new, tyre sizes vary as well, so not all the same marked tyre sizes are in fact the same size (eg; diameter).

Cheers
Gary

[Guest001]

Aside from GPS speed nothing will compensate for tyre wear affecting the distance travelled with each wheel rotation (ie; speedos and ODB outputs are simple rpm counters). Even brand new, tyre sizes vary as well, so not all the same marked tyre sizes are in fact the same size (eg; diameter).

Cheers
Gary

RPM doesnt come into it You could do a Km in 2nd gear and its just as far as doing it in 5th .But I think I know what you mean

Its impulses per kilometer in most electronically controlled cars today and mine is taken off one rear ABS sensor to tell ECU the number of clicks against a preset norm. To change it you have to change tyres or reset the ECU to accept a different reading per Km.

The difference between a new tyres reading and one that is nearly worn out is approx2.2 % of diameter so wont hardly upset the speedo, if anything will read slightly higher
ABS sensors also do this
Indirect TPMS use the ABS wheel speed sensors to detect wheels that are revolving at different speeds. An underinflated tyre will have a slightly smaller rolling radius and it's that that the car detects.

[Ozsko]

I dont think we need to worry about all the nuances

When talking about a few KPH that is exactly what it is. It was my understanding but may not be right that it was illegal to sell a car if the speedo did not read fast and that requirement was written into the ADR's.

[Guest001]

When talking about a few KPH that is exactly what it is. It was my understanding but may not be right that it was illegal to sell a car if the speedo did not read fast and that requirement was written into the ADR's.

See reply nine for correct reference to the ADR Rule and it only applies to cars registered after July 1 2006

Before that it was Wild West + - 10% of true speed

[Guest001]

This seems to be the solution maybe In Dash Board Long Coding there is a setting for Wheel Circ which gives 8 options Mine is set on 1

Wonder what it would do if I changed it to a different one..

Some models have setting called K setting which if you go up in the numbers it gives a slower speedo reading as it was supposed to in the Passat

[Ozsko]

RPM doesnt come into it

But total tyre roll out does and that will vary with brand and pressure, vehicle weight etc. Carry on and let us know what happens and how you referenced the calibration. The best way is using the surveyed length markers over a distance and not GPS. The surveyed length we have near us is 5 kilometres and clearly marked end to end by signs specifically for the purpose of speedometer correction.

[Guest001]

The circumference of the tyre is what makes the difference My 255/40/20 have a circumference of 2236mm and do 719.6 revolutions per KM.

To bring the speedo more or less correct it would take a 255/45/20 which is 2317 Circ and does 694 .5 revs per km which is 3.7% less.

All this is assuming tyre is pumped up to correct pressure and is new.

I might try a couple of adjustments and you dont need to worry about a worn tyre as its about 12mm smaller but only makes 2% faster speed approx difference in speedo

[Sydneykid]

RPM doesnt come into it You could do a Km in 2nd gear and its just as far as doing it in 5th .But I think I know what you mean

Its impulses per kilometer in most electronically controlled cars today and mine is taken off one rear ABS sensor to tell ECU the number of clicks against a preset norm. To change it you have to change tyres or reset the ECU to accept a different reading per Km.

The difference between a new tyres reading and one that is nearly worn out is approx2.2 % of diameter so wont hardly upset the speedo, if anything will read slightly higher
ABS sensors also do this
Indirect TPMS use the ABS wheel speed sensors to detect wheels that are revolving at different speeds. An underinflated tyre will have a slightly smaller rolling radius and it's that that the car detects.

I'm pretty sure I didn't mention engine RPM, the ABS sensors simply counts the revolutions of the wheel and multiplies that into KPH (Pythagoras in action) for example a 255/40/20 has a circumference of ~225 cm when brand new. Tread depth does vary between tyre manufacturers but mine has around 11 mm when new and the minimum is 2 mm, hence a 9 mm reduction in radius, which equals 217 cm circumference, hence 3% smaller than when new. Coincidentally mine has around a 3% conservative speedo with new tyres, so with just legal tyres that's cumulative 6%. Which means at an indicated 120 kph it's only doing a real world 113 kph (GPS speed).

In practical terms when visiting my son in Melbourne (I'm in Sydney) if I drove by the speedo I'd still be at Wandong instead of arriving at his place when driving by the GPS speed.

Cheers
Gary

[Guest001]

I'm pretty sure I didn't mention engine RPM, the ABS sensors simply counts the revolutions of the wheel and multiplies that into KPH (Pythagoras in action) for example a 255/40/20 has a circumference of ~225 cm when brand new. Tread depth does vary between tyre manufacturers but mine has around 11 mm when new and the minimum is 2 mm, hence a 9 mm reduction in radius, which equals 217 cm circumference, hence 3% smaller than when new. Coincidentally mine has around a 3% conservative speedo with new tyres, so with just legal tyres that's cumulative 6%. Which means at an indicated 120 kph it's only doing a real world 113 kph (GPS speed).

In practical terms when visiting my son in Melbourne (I'm in Sydney) if I drove by the speedo I'd still be at Wandong instead of arriving at his place when driving by the GPS speed.

Cheers
Gary

According to the tyre comparator I use the circumference is as above but whatever depending on the brand. Mine have 8.5mm after 2000km

RPM = Revolutions per minute. Not Revs per KM

But yes thats basically what I said isnt it. As a tyre wears it becomes smaller and so speedo gets proportionately higher than actual.
Anyway still havent got an answer on how to get it closer

[dgp]

According to the tyre comparator I use the circumference is as above but whatever depending on the brand. Mine have 8.5mm after 2000km

RPM = Revolutions per minute. Not Revs per KM

But yes thats basically what I said isnt it. As a tyre wears it becomes smaller and so speedo gets proportionately higher than actual.
Anyway still havent got an answer on how to get it closer

Did you try the long coding idea for wheel circ?