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The Natural Philosopher The Natural Philosopher is offline
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Default Graph of car fuel consumption versus speed

Chris wrote:
I would like to make a graph of car fuel consumption versus speed.
My driving is not smooth enough to gather the raw data myself.
Do you know of any reliable figures, or graphs?
I'm interested in relative values, rather than those for any particular
car.


I think that frictional losses including the rolling resistance are
pretty much linear with speed, but aerodynamic drag is the cube of
velocity. Or it might be the square.

Hence economy driving broadly falls into these categories

To reduce frictional losses - otherwise approximately constant per mile
- lighten the car and pump the tyres up. And fit eco-tyres. You can
probably get 3-4% this way

- keep speeds below 60 mph at which point aero losses start to mount
sharply. This is significant. On cars with consumption meters 50-70
represents about 10% increase in fuel consumption, over that it goes up
massively.

- strip all external junk like roof racks and the like. There is
probably at 70mph a couple of percent to be had here.

- try and drive at a gear and speed where the engine is most efficient.
For a diesel that is at the lowest throttle setting IIRC where the
fuel-air ration is leanest. That possibly means use revs and less welly
to get acceleration and power, not slogging in a low gear at higher
throttle settings. For a petrol it may well be the other way around I am
not sure. This can net you about 5% from typical driving styles.

- reduce acceleration and braking to a minimum by anticipating the road.
Braking represents a net loss of energy that is never recoverable. This
is as great a contributions as speed reduction. Especially in towns.