Not quite sure how you fathom that one out work but that's simply not true (unless you were going up a steep hill that is).
You say Ford know what they are doing by now - have you read their press release on this subject? Ford say they've fitted the 5spd because they expect the petrol Fiesta's to be used for town driving hence the bias towards higher gearing.
Well given the number of petrol Fiesta's I saw on the motorway today, I seriously doubt that's true. I'm sure the cost factor if not the lack of space is a more probable reason.
I've covered almost 500 miles today and regardless of what Ford say, a 6spd box would defo have been better for economy sitting at 70mph.
Speaking of Ford knowing what they're doing - I've only had the car for a few weeks but noticed something else that's weird. The car comes with a heated windscreen - brilliant. Yet the washer jets are exposed on top of the bonnet and neither do they look as if they're heated. Kinda defeats the purpose of a heated windscreen then if after a few minutes driving it's covered in salt and you can't do anything about it. Did anyone find their screen jets froze last winter?
50 mph in 5th is out of the power band for the engine so it has to work harder than if it was at 60 mph. do a practical test and you will see its true