Lots of factors here, but driving style is a good way to put it. When we did the drive from OH to TN (500+ miles each direction, over 1,100 miles in a week period) we found that we’d arrive with the same, up to, as you said, 5% extra — but freeway speeds tended to be closer to accurate. And this was during warm/summer months. We did 75MPH in mountains at various times, still getting net results that were at or above estimated.From my experience, it always estimate more than real.
Most of the case, it remains around 5% more than it's estimation.
Maybe it's due to my drive style.
I’ll be really curious to see how winter performance turns out for all of us, especially those of us in colder climates. Where I live (NE Ohio, snow belt) we will sit below freezing for a few months, and still do fair amount of driving. We have a big trip planned late December too, but south to a warmer climate. It will be very interesting to see if the estimate accounts for weather appropriately or not.