Speedometer calibration

hello everyone,

Okay, so the problem is:

With tires of size 185/60 R14 (rolling circumference: 1.763 m), the speedometer reading will be higher than the actual speed, meaning it will display a speed that is too high.

With the tire size 205/45 R16 (rolling circumference: 1.804 m), the displayed speed matches the actual speed exactly, according to the ADAC measuring device. Therefore, there were no problems registering the tires/rims.

'Now I have 215/40 R17 tires installed (rolling circumference: 1.843 m), and with these, my speedometer shows a lower speed than the actual speed... It's not really possible to get that corrected through official registration, and besides, it bothers me personally.'

I think I once read or heard somewhere that a wheel rotation generates 19 pulses. What exactly do these pulses look like? Or can someone tell me how I can figure out what these pulses look like myself? The only thing I have available is a digital voltmeter.

With larger wheels/tires, theoretically, more pulses would need to be delivered in the same amount of time because a greater distance is covered. So, perhaps 22 pulses instead of 19. How can this be achieved? It's not a problem to reduce the number of pulses (using a flip-flop), but how can we *add* pulses? Does anyone have an idea?

I used the search function, but unfortunately, I didn't find anything useful.

Thank you in advance.

Okay, it's a Golf III with an AFN engine, model year 1997.

Best regards.

BlauBaer wrote:

With larger wheels/tires, theoretically, more impulses would need to be delivered in the same amount of time, because a greater distance is covered.

Hi,

That's where the flaw in the reasoning lies.
The impulses are not generated per track, but per wheel revolution.
Therefore, your speedometer still receives the same number of impulses, but there is more distance covered for each impulse, which the speedometer doesn't account for, and therefore displays a lower speed than the actual speed.

No, no... not a misunderstanding... just a misinterpretation/misexpression.

At the same time, we're talking about the time it takes for one revolution, and then the distance covered in that revolution. So, for 1.763 meters, 19 pulses are generated, just like for 1.843 meters, 19 pulses are generated. However, for the 17-inch measurement, it's not enough, so I want to increase the number of pulses. Do you have any ideas on how to do that? The goal is to have more pulses reach the speedometer within the same time (one revolution).

Best regards.

Possibly another AI with a different momentum value? It can also be from a gasoline engine, as I recently discovered.

Best regards, WarLord.

Hello BlauBear,

I seem to recall that you were able to change the number of degrees of freedom of the AI by recoding it.
or that there was even a channel for customization.

That would be a bit simpler than generating additional impulses...

Best regards, Frank.

edit (Gremlin): made the link clickable

hello,

@frank, thanks for your help. I don't have a Golf IV where I can adjust that because the instrument cluster (KI) is connected to the bus. However, I still found something useful. It seems someone knows a lot about the EEPROMs in the instrument cluster.

Best regards.