scott wrote:

How do you work that one out? If I'm using more power to go faster, surely
I need to be using more petrol?

That depends how much more power you're using and how much faster you're
going.

As air resistance increases with speed squared, the amount of petrol used
goes up quite quickly once you get to higher speeds.

Assuming you're measuring by distance, the amount of energy used to
overcome air resistance is proportional to speed squared. However there
are other factors to consider. The amount needed to accelerate a car is
proportional to the speed you're accelerating to (minus the speed you're
accelerating from) and IIRC rolling resistance is proportional to the
distance travelled. Then there's the energy needed to keep the engine
turning (which is proportional to the time taken, so you use more if you
go more slowly.

However, the characteristics of engines vary widely - for each design,
efficiency varies differently according to speed, load, and how hard you
accelerate them. Until you know how they do, you won't be able to
determine anything much.