I guess you are checking your speed off of a gps.
If your speedo says 70 and your are doing 77...then you are actually about 9 mph off... Ford programs the speedo where its about 2 mph fast at 70 ( so if your speedo says 70 your going 68).
I just noticed you had 20's you might want to go a little higher than 2760mm because I believe they have a little less squish.
And what I forgot to mention earlier, when you are using the formula to figure out what number to put in for your tires...always remember to deduct 3%-5% for "Squish". Chances are you did the formula right, you just didn't subtract the squish.
Hope this helps
