Speedo Correction Is it possible

[Sydneykid]

A bit off topic but what do you mean, you use a physical speed sensor corrected to read the same as GPS?

It's a correlation, as the tyre grows with rpm (centrifugal force) using a wheel speed sensor always results in some errors throughout the speed range eg; if we correct the speed for say 80 kph, then it will be conservative at 250 kph. Some data acquisition software allows us to build a compensation table for tyre growth. Plus during a race we suffer from tyre wear and even with the limited tread depth it still introduces an error as the tyre diameter decreases. There is also wheel slip (eg; wheel spin on the driven wheels, lock up under brakes etc) to consider. On most cars we run 4 wheel speed sensors via an algorithm in the data acquisition software to mitigate those issues. As a result I use a correlation between the speed based on the algorithm of the wheel speed sensors and the GPS speed. The line of sight (to the satellites) at race tracks is usually pretty good, so the only issue with using GPS data is the time delay, which we can compensate for in the software.


Cheers
Gary

[Sydneykid]

So have just been for a run down the motorway and here is the result Changed as above from 1 to 3 and at 100kph the OBDII says I am doing 100kph where as on the 1 setting I was doing 104 on the speedo when OBD II said i was doing 100

So what you have done is to align the speedo with the ODB output. That doesn't mean that they are correct (ie; show the true speed) just that they are now the same.

Cheers
Gary

[Hillbilly]

So what you have done is to align the speedo with the ODB output. That doesn't mean that they are correct (ie; show the true speed) just that they are now the same.

Cheers
Gary

According to the experts on the OBD dongle it reads the speed off the ECU similarly to what the Passat did in the extra menu and so is supposedly the true speed..

Tiggers reply no 3 seems to confirm this

No I didnt I altered what it said to do to make it more accurate and then checked with the OBD AFTER having previously read one against the other and it told me it was 4 fast as did the Tomtom i had with us at the same time

I dont really care Im not going to measure the tyres every time i get in it, All I wanted was it a bit closer to true speed. Tomorrow Im going to take a GPS and check it against that.
You can measure your race cars to the 9th degree but we dont want or need to do that.

Im not making a crusade out of it I thought it may be helpful to others but seems maybe not accurate enough for some.

[Sydneykid]

According to the experts on the OBD dongle it reads the speed off the ECU similarly to what the Passat did in the extra menu and so is supposedly the true speed..

Tiggers reply no 3 seems to confirm this

No I didnt I altered what it said to do to make it more accurate and then checked with the OBD AFTER having previously read one against the other and it told me it was 4 fast as did the Tomtom i had with us at the same time

I dont really care Im not going to measure the tyres every time i get in it, All I wanted was it a bit closer to true speed. Tomorrow Im going to take a GPS and check it against that.
You can measure your race cars to the 9th degree but we dont want or need to do that.

Im not making a crusade out of it I thought it may be helpful to others but seems maybe not accurate enough for some.

I apologise if it seems that it was being obtuse, I honestly was simply pointing out that any speed reading that uses a revolution counter (whether it's the speedo or the ODB output from ECU) is still subject to the same variabilities of tyre diameter.

Cheers
Gary

[Hillbilly]

I apologise if it seems that it was being obtuse, I honestly was simply pointing out that any speed reading that uses a revolution counter (whether it's the speedo or the ODB output from ECU) is still subject to the same variabilities of tyre diameter.

Cheers
Gary

Today I took the GPS and the OBDII and went for a trip down the motor way Got it up to 100 all three read 100 so thats near enough for me and probably most of us.

Yes I understand that circumference diiferences will give different readings and thats probably why it is adjustable in the dashboard readout As my model can have 4 different tyre sizes as standard and maybe that what they use to allow for that.

However the average Joe doesnt need really to worry that his tyres on a hot day may expand by 3mm and put his speedo out by less than1%.

Also if they are at the optimum pressure it should only increase by about 4lb and so wont make hardly any difference will it.

If anyone wants t do it thats my experience. If no one wants to do it thats fine as well