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I'd think values above 22 are fine, above 25 are very good, above 30 excellent! Mine does around 24 mixed driving, exept bad weather. Didn't hear of additives lowering the average during cold season here - except for diesel.
Do you remember old cars having a manual choke? (at least we call it that way in Germany, had it till late 1980s) This causes the engines to run rich. I bet the modern cars do exactly the same thing automatically to run smoothly. I guess the biggest part of lower gas milage is caused by engine running rich for a longer time during cold season...