These days, having an accurate car speedo is vital if you’re to avoid fines and loss of licence points. But how do you correct the speedo if it is reading high or low? It’s easy with our Super Speedo Corrector which will work with any electronic speedo, either digital or analog. It allows you to alter the speedo reading in 1% increments, either up or down.

Fig.3: all the clever stuff in this circuit is done by PIC microcontroller IC1. It takes the speedo signal and multiplies it by a factor set by two rotary BCD switches (S1 & S2). The speedo signal frequency can be either increased or decreased in 1% increments - see text.

Before you can use the Speedo Corrector, you’ll have to find and identify the speedometer sensor output wire or the speed signal output wire from the ECU. In some cars that’s easier said than done, so make sure you have a wiring diagram and that you can physically access the speedo input wire which is 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.

This project is a development of the Speedo Corrector first published in the SILICON CHIP book "Performance Electronics for Cars". Advantages over the original project include an automatic set-up procedure where the Super Speedo Corrector calibrates itself to suit the speed signal output characteristics, an on-board status LED that flashes to show correct operation and an AC output signal that will work with Nissan speedometers.