Given most factory speedos read high you might be a lot closer to actual. I'd go do a couple of runs with a gps speedo and see how it compares before I started tutuing with electronics.
Step one would be ascertaining how it gets its speed. The primary source is the speed sensor on the gearbox, but it might reference this against your wheel speed sensors and having a discrepancy could make your ABS rather unhappy. Being a MY04 it's presumably still cable throttle (IIRC DBW cutover was '05?) so you won't have complex VSC/Traction Control which will get very s****y if the gearbox output is different to the wheelspeed.
As to how you'd go about calibrating it, much harder. I guess you could build some sort of brick that took the pulsed input and output a scaled version of it? I suspect the frequencies involved would be pretty mundane in the world of signal processing electronics, but the circuitry and maths involved is pretty cursed. Good starting point would be to get a scope on the output of the sensor and see what frequency and waveform you're dealing with.
By
boon ·
