I don't think I have seen a speed camera on a bit of open motorway. They only tend to be where there are speed limits in urban area's and around roadwork’s.
My point was where there is a defined speed limit - 20, 30, 40, 50 then it’s simpler to just stick to the speed limit and I think you will find that my rule works every time because this is were the congestion is, also your multiplier rule does not work in these instance (e.g. 10 miles at 30mph is 20 mins, 10 miles in a 30mph zone at 45 saves you about 5 to 6 mins at best on a clear run). These are the environments the speed camera’s are in.
Also, I think you are deluding yourself if you think your average speed over a whole 280 mile journey from door to door is 70 mph. To get an average speed of 80 MPH on an average 280 mile journey from door to door with 5 or 10 miles of urban travel at each end would mean you would have to be doing well over 100 on some sections of the route
. I find it hard to get averages over 60 MPH on such journeys even when I have been “tonking on a bit” in between - fuel computers are wonderful things they really do make you take a fresh look at you driving habits particularly when you check your averages.
I have been driving for 25 year and in the days before speed camera have done virtually every speed you can imagine (in a standard road car) and this is just experience talking. By driving fast you just think you are getting there hours before everyone else when in fact its only minutes at best, hit a bit of congestion and everyone arrives together – the only difference then is the number of points on your licence.