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Piwoslaw 04-16-2017 08:30 AM

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...

nemo 04-16-2017 03:14 PM

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

Piwoslaw 04-16-2017 03:31 PM

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...).
Quote:

Originally Posted by nemo (Post 538654)
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 04-16-2017 04:17 PM

Quote:

Originally Posted by Piwoslaw (Post 538655)

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.:eek: Maybe there is some way double the signal going to just the speedo.

Piwoslaw 04-17-2017 01:35 AM

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 04-17-2017 07:05 AM

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 04-17-2017 09:22 AM

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

seifrob 04-17-2017 02:07 PM

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 04-17-2017 11:38 PM

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 04-18-2017 01:19 AM

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.

hamsterpower 04-18-2017 06:51 AM

This is a common problem in the 4X4 world as they massively change the size of their wheels, thus the calibration of their speedo/odo. There is a "blackbox" that can be plugged in between the speed sensor (VSS) and the ECU. The "blackbox" has adjustment screws to change the calibration. Unfortunately, I can not remember what that box is called.

Stubby79 04-18-2017 09:53 AM

Just search for a VSS box or VSS converter. Easy enough to find.


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