Speedo Correction Is it possible

[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

[Hillbilly]

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.

[Hillbilly]

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

[Hillbilly]

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.

[Hillbilly]

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

[Hillbilly]

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?