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Old 04-16-2017, 08:30 AM   #1 (permalink)
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Rescaling the speedometer

I remember a time when car speedometers ended at 180 km/h, our P307's speedo goes to 210 km/h. I've only gone to 150 once, when we bought the car, since then I try to keep my speed below 100-110 km/h.

We are looking at getting a new car, and almost all of the models I've looked at go to 230 km/h, even though I searched for "eco" models.

Why do I need a speedo that goes almost twice as fast as I'll ever need to? Except for the Autobahn, most highways in Europe are limited to 120 km/h (140 in Poland and Bulgaria). Why not re-scale it to 0-150 to get better resolution?

Question is: How would I go about doing that? It's not enough to print a new background, I need to reset/recalibrate the step motor (or whatever it is) that controls the position of the speedo indicator needle. How?

Then there is question of how legal the new setup would be...

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e·co·mod·ding: the art of turning vehicles into what they should be

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[Old] Piwoslaw's Peugeot 307sw modding thread
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Old 04-16-2017, 03:14 PM   #2 (permalink)
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I would think the first thing you would need to know is how the speed of your vehicle is determined.
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Old 04-16-2017, 03:31 PM   #3 (permalink)
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When comparing to a GPS, the stock speedometer shows 2-4% higher than the actual speed. When I use an OBD tool (ScanGauge) to read the speed, it is spot on with the GPS, not with the speedometer(strange...).
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I would think the first thing you would need to know is how the speed of your vehicle is determined.
Most likely a sensor on the transaxle (when I floor it and the wheels spin, then the speedo goes up).

Which gives me an idea: add a second magnet to the transaxle, so the speedo shows twice the speed, then re-scale the speedo's background. Unfortunately, this would double what the odometer logs
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e·co·mod·ding: the art of turning vehicles into what they should be

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[Old] Piwoslaw's Peugeot 307sw modding thread
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Old 04-16-2017, 04:17 PM   #4 (permalink)
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Quote:
Originally Posted by Piwoslaw View Post

Most likely a sensor on the transaxle (when I floor it and the wheels spin, then the speedo goes up).

Which gives me an idea: add a second magnet to the transaxle, so the speedo shows twice the speed, then re-scale the speedo's background. Unfortunately, this would double what the odometer logs
That would be a major downside. Maybe there is some way double the signal going to just the speedo.
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Old 04-17-2017, 01:35 AM   #5 (permalink)
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I think that the signal from the speed sensor is collected by the ECU and translated to PWM for the speedometer. This means either I would have to catch that signal between the ECU and speedo and change it to something different, or somehow change something in the speedo itself.
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e·co·mod·ding: the art of turning vehicles into what they should be

What matters is where you're going, not how fast.

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Old 04-17-2017, 07:05 AM   #6 (permalink)
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Gut the speedo and fit a nice modern GPS speedo back there.

Universal GPS Car Speedometer HUD MPH KM/h Plug Speedometer Overspeed Warning#IR | eBay

You can get nicer ones if you want to spend more.
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Old 04-17-2017, 09:22 AM   #7 (permalink)
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Old 04-17-2017, 02:07 PM   #8 (permalink)
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If its just a whim, its imho not worth the effort. You could tap the speedo PWM signal, process it in some microcontroller (arduino, picaxe...) and send forth, but...
Why not to utilize big-screen MPGuino, Torque Pro app or something similar, if all you need is more precise speed info?
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Old 04-17-2017, 11:38 PM   #9 (permalink)
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It is just a whim, just a thought-experiment on how it could be done. I really have no intention on playing with PWM...

But thanks everyone for the feedback
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e·co·mod·ding: the art of turning vehicles into what they should be

What matters is where you're going, not how fast.

"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell


[Old] Piwoslaw's Peugeot 307sw modding thread
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Old 04-18-2017, 01:19 AM   #10 (permalink)
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If a car dashboard were one big LCD screen and not dozen of individual light bulbs, LEDs, dials and other goodies, it could be a mere constant adjusting.

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