It's alright, and there is some data to evaluate albeit from a skewed reference point, but and at least it will be the same reference.
I'm given up after 8 days in trying to beat anything more than 65 on my shortish (max 20 mile) journeys, up and down hills, in traffic around windy country roads. The big killer is the gear changes. What a load of s**t!
However, if you have few moments to compare identical journeys say once a week or if it's a commute, you can perhaps look at a trend over the weeks/months to see if there is any significant performance drop, or pre/post service etc, carrying passengers, ECO on ECO off.
Just saying, that's all..I'm not recommending it!