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The basic effects of a badly grounded MAF is the engine running like a 1960 Mini cooper - i.e. wandering, no power, crap economy, sounds like a bag of spanners, etc, etc.
Voltage measurement is relative between the ground pin and output pin: therefore if your output pin is reading 5v and ground pin is at 0v, then the ECU thinks it sees 5v. If, however, the ground is at 17mV, then the ECU sees 4.983V, not 5 (5v - 17mV). It may not sound much, but bad grounds cause all sorts of problems, few of which are actually related to dc voltage - current loops, emc pickup, frying ECU's (oh yes indeed), etc, etc...
The ECU doesn't adjust for economy, it adjusts for the best drive. It tries to balances a lot of different inputs - some on short time-scales, some on long time-scales. The o2 sensor is a long-timescale measurement - i.e. the ECU will gradually adjust to suit over a length of time.
Fuel economy is also a balancing act between getting every possible tune-up item correct. To be honest, anything can cause bad economy - including the oil filter!
If you want any more information, let me know, but it gets horribly complex for here on in...
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