My average MPG is consistently 2 to 3 miles above my actual average. Every time I fill up I record and then reset the Average MPG on both trip meters. Then, I calculate the actual. The difference between what the truck computer says it got and what it actually got is consistently between 2 and 3 mpg. The truck says it does better than it actually does. I've experimented with all three octane ratings with no apparent effect on mileage performance.
As an aside, even on the open road at about 71 or 72 mph, I've yet to get more than 17 mpg actual. The car only has 8,500 miles on it, and I'm told the mileage performance will get better as time goes on. That improvement, if it occurs, will no doubt be more than negated by rising fuel costs.