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Speedo Corrector

Get your electronic speedo reading accurately!

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Cheap electronic kit
  • Correct inaccurate speedos in standard cars
  • Correct inaccurate speedo caused by changed differential or gearbox ratios
  • Correct inaccurate speedo caused by changed tyre diameters
  • Intercept and modify the speed signal, eg to power steering weight control or auto trans controller
  • Also corrects tachometers
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Changed your tyre diameter? Fitted a different diff ratio? Done an engine gearbox transplant? In each case you’ll need to correct the speedo reading. That used to be an expensive and difficult exercise but if you can build a simple electronics kit, you’ll now be able to do it much more cheaply. And accurately as well – down to plus/minus 1 per cent correction! That figure makes this design suitable for correcting even standard speedos – after all, who wants an instrument that reads wrongly?

Click for larger image

But before you can use the Speedo Corrector, you’ll have to find the speed sensor wire. In some cars that’s easier said than done, so make sure you have a wiring diagram and you can physically access the speedo input wire, normally at the back of the instrument cluster. If you can’t find the right wire, you won’t be able to install the Speedo Corrector.

The Speedo Corrector kit is available from Jaycar Electronics and costs just AUD$50. (Don’t know anything about building electronic kits? Have a look at Building Electronic Car Kits. The Speedo Corrector is of medium level difficulty).

Construction and Connection

Follow the instructions provided with the kit – if you’ve ever before built an electronic kit, it’s straightforward. The Speedo Corrector has just four connections to the car – ignition switched power, earth, speedo signal in (from the speed sensor) and speedo signal out (to the speedo).

Making Speedo Adjustments

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The Speedo Corrector has two multi-position switches to allow you to make speedo corrections. The speed reading can be altered in 1 per cent increments. This is most easily explained if you use a test speed of 100 km/h. If the speedo is wrong by 5 km/h at 100 km/h, the adjustment needed is about 5 per cent.

S1 (the multi-position switch nearest the bottom) corrects the speedo reading in single units and S2 (the other multi position switch) changes the output in tens. So where you want a correction of 5 per cent, simply set S1 to ‘5’ and S2 to ‘0’. If the required correction was 16 per cent, S1 would be set to ‘6’ and S2 to ‘1’. Using the two switches in combination allows the speed reading to be altered by as much as 99 per cent – or as little as 1 per cent. And everything in between!

Note: it’s easy during set-up to forget that the speedo correction is in per cent NOT km/h.

The default output slows the reading of the speedo. This is because most speedos read fast (often by about 5 per cent). But if you wish to increase the speed shown on the speedo, set Switch 2 to its F position and wait for a 2 flash acknowledgement from the LED. This needs to be done with the unit connected and powered-up.

In short, correcting the speedo output - either making it faster or slower - is straightforward.

To set the speedo you will need an accurate reference. This can be provided by a handheld GPS, another car with a known accurate speedo – or even, if you ask nicely, a police car. (Just make sure that you have an assistant to do the adjusting as you drive!) You can also use the ‘speedo check’ distances that are marked on some roads – although strictly speaking, this is checking the accuracy of the odometer rather than the speedometer.

Mechanical Speedo?

The Speedo Corrector will work only on electronic speedos, that is, those that don’t have a mechanical rotating cable driving them. However, note that some mechanical speedos have an electronic output that sends speed information from the speedo to the ECU, so if you want to alter the ECU speed input, you can still do so. But it won’t change the speedo reading.

Installation

In the vast majority of cars little set-up will be needed – the corrector will mostly work out for itself what configuration is required to suit both the speedo sensor and the speedo. The steps to follow (which comprise certain switch settings followed by driving) are listed in the instructions.

Digital Speedos

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If the Corrector is fitted to a car with a digital speedo, some lag may occur in the action of the speedo. Typically this is noticeable when abruptly coming to a stop from a slow speed (eg 10 km/h), where the speedo may keep displaying a number greater than zero for up to a second, even when the car is stationary. Lag may also make itself evident when moving away from a standstill, where for example the speedo initially shows 0 km/h before then jumping to 15 or 16 km/h. However, this problem can be overcome by the use of the special ‘digital speedo function’ built into the Corrector. This function is enabled during set-up.

Tacho too?

The Speedo Corrector will also work with electronic tachos that take their feed from the ECU (ie all cars with engine management). The configuration procedure is the same as for use of the device as a speedo interceptor, except the ‘speed sensor’ becomes the tacho output signal from the ECU. This application is particularly suited to engine and gearbox swaps.

Conclusion

Once the Corrector is working properly, it can be mounted in its UB3 box and then tucked up behind the dash out of sight. But don’t then assume that your speedo is then always going to be dead accurate – accuracy depends on tyre diameter, which changes with wear and when new tyres are fitted. Of course, it’s easy enough to then make the required speedo calibration change!

Contact: www.jaycar.com.au- cat no KC5435

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