In some posts people complain that on their next trip after a mofification the MPG change they were expecting did not happen or went the opposite way.

One thing I learned doing MPG tests from Durham NC to Wilmington over a mostly flat I40 in a Ram was that
side winds,
Air temperature, and especially
pavement changes from blacktop to concrete
changed fuel economy WAY MORE than I first “guesstimated.”

To really do meaningful tests on real world interstate highways to nail down small changes on the order of 5% you need two vehicles travelling convoy style.

As a miner I know there are many miles of big tunnels in Limestone quarries and Salt mines. I once joked with a friend of my father’s who got out of the coal business and into a Limestone quarry that I wanted him to pave ten miles of it for me to do MPG tests.