Camaro, you need two input signals for MPGuino one is the vehicle speed sensor and second are pulses from a fuel injector. You've asked if you can use ABS sensor but I think it's not the best idea, because of the pulses count per revolution. MPGuino can lose some pulses and then the output will be not in correlation to reality. So the best way will be to glue one, two, maybe four magnets to the main shaft of your Camaro and put there contactron or a hall sensor. And you have constant number of pulses per mile, and that's the idea of VSS.
Second wire you have to connect to one of the injectors and then you have amount of fuel, and rpm signal for MPGuino.
That's all you need, to make your MPGuino working.
And a simple diagram for you:
1. +12V
2. VSS signal for MPGuino
3. 0V
4. Resistor ~10kohm
5. Contactron
6. Magnet
7. Main shaft
8. Diff