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Some 1990's General Motors ECMs have a disabled "highway mode" that leans out the fuel mixture a bit under steady cruising with part throttle.
The reason why it's disabled is it raises the NOx level a bit too much to suit CARB, so rather than disable it only for California destined vehicles, GM shut it off for all.
I'm going to have a PROM programmed for my 1982 GMC flatbed with it enabled and any other MPG improving program changes there are, that won't reduce power when it's needed.
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