This has wondered pretty far off the original post but we've gotten into the territory of something I've wondered about for some time.
I went back and looked at the trace (I have a Garmin) from my ride from Vesuvious up to the Blue Ridge Parkway. The exact points are hard to grab and I never corrected the altimeter function but I picked a point at the bottom but where it was already showing as steep and it read elevation 1500' and milage 22.08 and then I picked a point near the top but it had not leveled off yet and it was elevation 2865' and milage 25.00.
So you've gained 1365' in 2.92 miles. You plug that into your handy/dandy incline calculator:
and you get a incline of 8.8%,(which seems awfully low for that hill)
It was 2 years ago now so I don't remember exactly but the Garmin was reading 10 - 13% the whole way up that hill. And this is not an isolated incident, whenever I'm on something steep and I check to see what the read out is on the Garmin and then cross check it against the incline calculator, the Garmin readout is 2-3-4% high.
Now, the math is the math. I'm sure the incline calculator is giving you the right % for the numbers you put in. But the Garmin is doing it's own math.
It's human nature. When you say that it was reading 10-13% all the way up the hill, that means that it was reading 10-13% every time you looked at it. But when did you look at it? When you were curious, of course. When were you curious? When the hill was the steepest.
This is why we think hills are steeper than they are, why we think headwinds are worse than they are, why we think we ride faster than we do, etc.