Speedo Correction After Diff Change - Facelift TT Six Speed

[dandan]

I've just changed my diff and final drive ratio (from 3.266 to 3.538) and I need to correct the speedo inaccuray as it's reading ~7mph too fast.

 

What's the best way to do that?

 

Cheers

Dan

[TheTurtleshead]

I'm in exactly the same situation, so i'd like to find a way of sorting this as well. I was thinking of using a potentiometer of some kind in a signal wire, haven't investigated it at all yet. I have heard that you can buy a speedo converter with a adjusting potentiometer, needs googling though!

[johnny g]

Is it possible to have the Syvecs output a speed signal to the speedometer, based on diff and tyre sizes?

[Digsy]

You couldn't do it with a pot because the speed sensor signal is a digital pulse, not an analogue voltage. I did used to have a picture of the waveform but much to my annoyance I lost it.

 

You would have to build a small circuit (maybe PIC based) to sample the input signal and then output a duplicate waveform but with the frequency scaled to correct the error. IIRC the Thor speedo converter allowed you to do this within certain limits.

 

It does raise an interesting question, though. It means that the speedo head and ODO circuit must be available in different versions to match the diff type. I wonder if there is a simple change that can be made inside these components that tells it which final drive ratio you have (kind of like a jumper setting on a PC motherboard)?