Rescaling the speedometer

[Piwoslaw]

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...

[nemo]

I would think the first thing you would need to know is how the speed of your vehicle is determined.

[Piwoslaw]

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...).

Originally Posted by nemo

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

[nemo]

Originally Posted by Piwoslaw

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.

[Piwoslaw]

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.

[oldtamiyaphile]

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.

[jakobnev]

Black elctric tape on all the numbers and lines after 150.

[seifrob]

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?

[Piwoslaw]

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

[seifrob]

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.