The average mpg calculator works so quickly that you just have to hold a speed for about 1 minute after resetting the average and you'll get your "instant" mpg at that particular speed. If you want any more than that get the Scangauge E for about $95 online.
The '12 Focus on the other hand takes so long to calculate average mpg (like 5 minutes or more) it's impossible to hold one speed on a flat surface long enough to get your actual mpg at any give speed. That's why I've got a Scanguage on the way...