I used to have a 20 mile commute four times a day on dead flat roads. I bought a super-efficient car (skoda fabia greenline} and played with the speeds and gears to my hearts content to find the scientifically most efficient speed to use the least fuel. The result was that the slowest speed the car could maintain in the highest gear was most efficient. That was 30mph in 5th. I would usually average over 100 miles per gallon of diesel. Spent a lot more time in the car than i would have at 60mph, but audiobooks are great. This doesn’t add much to the discussion here. 60mph is much better than 70mph, and i’ve driven in wales with the 20mph limit in all urban areas and it doesn’t seem to cause any problems - i’m totally in favour.