I love it when the answers are "simple"!
On your speed. You really want to adjust the tire size so you dash speedo reads about 2 mph (about 3%) fast at 70 mph. So, if your GPS reads 70, your dash should read 72 and the Gryphon should read 70. Why? Because that's the way Ford calibrates things to make your odometer accurate. Why?
So anyway, you should go ahead and bite the bullet and open the Custom Options Menu. You're going to have to "play" here. But, I'd activate Pegasus, enter your tire size in the "old" tire size window of the Gear Ratio Calculator, then change the "squish" to 7% (this seems to work for me) and read the Tire Size (TS) in the "Old Circumference" window. Try it. If it doesn't work, increase the size if your speed is being reported slow or decrease it if it's being reported fast.
Your error is actually pretty minor though. And if you didn't have a programmer, you and everyone else would not notice it.
The "reported" Average Economy" is probably not too accurate. I find mine is higher than a "hand calculated" value at fillup. But, you MAY really get better mileage by getting to a higher gear more quickly than "feather-footing" it from a standing start. Naturally, things are relative. I suspect if you "floored" it, you'd get worse mileage.
Your observations make sense to me - and good reporting of those observations!
- Jack