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A couple of factors come into play in an ECM while in EGR mode:
Spark timing vs. EGR rate (Injection timing on a diesel)
Rate of EGR vs. engine load
EGR flow diagnostics.
This has been stated already I think, but to clarify: The EGR by itself does not add fuel economy. What does add fuel economy:
-The ability to run advanced timing while in EGR
-The need to open the throttle slightly with EGR, reducing pumping losses.
I have added 2-4 degrees of extra timing during EGR event in ECM programming, with adverse effects. Unfortunately I have no evidence of whether or not this change improved fuel economy.
Interestingly, the Duramax diesel usually shows a slight improvement in economy with the EGR completely disabled.
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