What if the PCM mathmatically averages all instantaneous mpg readings and displays it as "Average MPG". In that case as the number of crazy instantaneous mpg readouts increased so would the average mpg.
In other words if you coast a lot or back off the throttle a lot, as can happen when not using cruise control, your average would be greater. If you're doing 75 mph and reduce speed to 70 your instantaneous mpg increases dramatically. This in turn results in an increased average mpg. The mathmatical average of a bunch of 99's is greater than a bunch of 19's.
