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Cal table for speedo / tacho


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My car is 30+ years old, so things like the tacho and speedo dont work quite how you'd like. Especially when going RPM and speeds well past what was ever intended haha.

So my problem is that if I calibrate the speedo at 100kph (or 50kph) it loses accuracy either side.
I've tried fiddling with offset / multiplier etc a bit but I think it's because it uses an old style mechanical return spring that's a bit worn out so is less linear across the range than you'd hope.

I could set a test PWM function and record the hz at which various key speeds are being hit. Then have say a 3 or 4 point cal table which helps maintain accuracy right from 0kph to ~180 or so if it was possible to input this info into the speedo calibration.

Otherwise if I could link a general purpose PWM output to a 2D table that varies the hz at which it operates then I could manually configure it that way instead of the speedo function (Is there a way to do this?)

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Hi Davidv,

I have never had to play with something like you are trying to achieve, however I just wanted to make sure that as well as your ideas  that you knew about the following settings in the software also.

The driven wheel setting can obviously be derived via an electronic speedo transducer or mechanical to electronic transducer to help with a more accurate input as far as the speedo is concerned.




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Thanks Dave.

The speedo signal that the ECU gets is accurate as per GPS. As I use it for gear detection / gear dependent shift light rpm.

It's just that when displaying it to the factory gauge in the dash, if I calibrate it at say 100kph it pretty quickly loses accuracy when I'm going say 50kph or a bit faster at the track.

Same for tacho, if I adjust the settings so that when the car is idling or mid rpm I can get the needle reading accurately. Then when the revs are high it is completely off target.

Hmmmm maybe it would pay to actually log down some notes of what the indicated differences are, and then see if it follows a pattern that might be fixable by adjusting the offset or duty cycle in a way I havent tried.

Edited by Davidv
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Thanks Mapper,
I initially had a look at the GP PWM but didnt see the table attached... I thought you could only statically set a HZ at which it runs at, and setting the switching criteria. (Which would be odd!)

It should be easy enough to calibrate, I think should work great, thanks for the suggestion.
I'll give it a try over the weekend and report back on how it goes.

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  • 2 weeks later...

I thought I'd better get some data to show what I'm meaning in terms of wandering accuracy etc.

So I setup a test PWM and adjusted the frequency until it matched certain rpm / speed points on tacho and speedo.

If they were accurate with a linear increase in hz then the graphs would show a flat line instead of rising with speed / rpm.



Based on above it looks like if I aim to get my speedo accurate at the 80kph mark it should stay accurate-ish from there up. But will lose accuracy as speed drops.

The tacho looks like a lost cause though, which based on experience it just flaps around fairly meaninglessly once the revs get up there haha.

But with a non linear calibration table it would be possible to get them both reporting bang on accurate. :)


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  • 5 months later...

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