On 09/09/2012 17:57, Bill wrote:

A displacement boat has a "natural" maximum speed (the hull speed)
dependent on the hull length. I've often wondered if something similar
applies to cars in air.

It might explain why in the old Omega, fuel consumption on the motorway
only went skywards over 70, but my current vehicle is bad over 60.

Or it might not.

Hull speed is when you are travelling at about the same speed as a wave
of the same length. Increase speed and your stern wave falls behind, so
you are climbing a wave at the front but not getting pushed by the stern
wave behind - and power needed goes up.

In a car air resistance is proportional to the square of the speed. (and
power to the cube). So go faster, use much more fuel. And you get odd
effects in engine efficiency too - petrol engines are really inefficient
at low power settings, so go a little faster and you may get better fuel
economy especially on a large petrol engine. I'm guessing the Omega
wasn't a diesel? And that the current car has a smaller engine?

Andy