[SentraSE-R]

You're averaging out most of the variables, and doing a good job, from your description. But we're trying to measure a 2-3% change, and all it takes to toss that away is a couple of stabs on the brakes when someone merges in front of you, or one stomp on the gas pedal to pass someone going slower, on your daily commute.

I used to be a backup driver in a vanpool, doing the same route every day. Most days, our afternoon drive was an hour and ten minutes, but one accident, or one slow driver on the hills and curves, could turn any day's commute into a 2 hour drive, and one day/year on average, the sea would part, and we'd find ourselves home in 45 minutes. Tank to tank variability was well over 5%, regularly.