Historically it was a well known stated thing that 56mph was the sweet spot between the gearing, engine speed and air resistance. Going slower means lugging the engine if you over-gear and is less efficient because covering less ground for the engine speed, therefore fuel burnt. 20mph will not save fuel. Electric cars will maybe get better range, but they may not, because every motor has a design efficiency for a certain rpm and the designed gearing to the roadspeed from the manufacturer.
Historically it was a well known stated thing that 56mph was the sweet spot between the gearing, engine speed and air resistance. Going slower means lugging the engine if you over-gear and is less efficient because covering less ground for the engine speed, therefore fuel burnt. 20mph will not save fuel. Electric cars will maybe get better range, but they may not, because every motor has a design efficiency for a certain rpm and the designed gearing to the roadspeed from the manufacturer.