Economical Speed

[slawit]

When I started driving 25 years ago the optimal speed for economy was around 57mph. Is this still the case in modern cars, also can someone explain to me the physics behind an optimal speed.
Slawit

[fredric21]

The economical speed for a land vehicle is dependent on the following factors:
Engine efficiency at low power, tire rolling resistance and air resistance.

Rolling resistance varies with vehicle weight and tyre contact area. The greater the weight of the vehicle, the more the rolling resistance due to an increase in contact area. More tire pressure reduces rolling resistance, because it reduces the contact area.
Air resistance varies with the frontal area of the vehicle and the square of the velocity. Twice the speed equals four times the drag, whilst four times the speed equals sixteen times the drag! The frontal area directly affects the Coefficient of Drag, as does the shape (although to a lesser degree).
An exact figure for the most efficient cruise cannot be given.