This has seemed to be fairly accuarate:
MPH / (RPM * Inj Pulse Width * Inj Constant * 0.000212)
Inj Constant is the rating of your injectors in lbs/hr
The pulse width can be retrieved via most GM ALDL interfaces, same with mph and rpm.
You HAVE to average this over say, 2-3 minutes because it jumps around alot depending on if you have the car floored or no throttle at all.
This is only valid for a car with 8 injectors, and I believe only for batch fire cars as that would change the 0.0000212 constant otherwise.