DIYbanter - View Single Post - Graph of car fuel consumption versus speed

The Natural Philosopher

AJH wrote:
On 08 Oct 2008 10:25:39 GMT, (Andrew
Gabriel) wrote:

I suspect engine,
gearbox, etc has probably been designed for good fuel consumption
at higher speeds instead, and this is more than countering increased
drag up to 65MPH.

I'm sure you are right, the engine is probably the first thing
optimised for this speed, below the optimum the thermodynamic
efficiency tails off significantly with a petrol engine, less so with
a diesel.

A smaller engine in the same car should move this part of the graph
toward higher mpg. I think this is one of the reasons the original
hybrids appeared poor in open road conditions they had smaller engines
and did best at about 45mph on the open road.

With an average drag I think the rolling resistance equals the wind
resistance at less than 20mph (12mph for a cyclist) and intuitively
this point is the best combination of energy efficiency and
timeliness.

Highly debatable.

Light skinny cars with skinny wheels and small engines probably peak
there, but heavier cars with fatter tyres and bigger engines probably
peak - albeit at far worse consumption figures - somewhere in the 50-60
mph mark.

AJH