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Prior to connecting directly to the injector and VSS input at the ECU, I was using an ALDL setup to try and get the MPGuino working that way (I thought it'd be a cleaner setup).

I started sampling the VSS sensor at the ALDL connector, and after some testing, concluded the value was 4750 pulses per KM.

I pondered more during my trial run but concluded that any request / response setup would be inherently flawed by the timing of the requests, interrupts ftw.

Finding the ECU, one time digging around, I thought we'd tap those connections. So I looked up the 4G63 ECU pin out online, and found the pinout - VSS and Injector one are taped into with jumper wire to my temporary setup.

I tested the value 4760 in the MPGuino, and ran a tank through it, the MPGuino thought we had done 500KM when we did 484.0KM.

From the start of the tank to today, I did random calculations to see how my value was travelling - it seemed wrong as the random tests gave numbers between 4950 and 4969. Most commonly, 4955.

I corrected the value to 4955 today. Drove ~6.58KM according to the cars ODO - the MPGuino logged 8.85KM. This disappointed me, the fuel economy was logging 8.2L/HK. Damned.
The calculation now suggests a value of 6664 - way too high.

If it helps, it's a 5V signal on that, as I tested it with a multimeter.
I did use a 47K resistor, and a Zener diode.

Any ideas on why the VSS value seems more dynamic than expected?
Any testing / troubleshooting suggestions? I had hoped to have an accurate setup so I could start testing the suggestions given earlier...