"Adrian" wrote in message
...
Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough rule
of thumb, increasing drag starts to come seriously into play from about
60mph upwards.

Drag cubes with velocity and so it may become important at speeds lower
than 60 mph.

Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.

Well it's present at any speed and where we determine it becomes significant
is actually a function of how large the drag force is compared to the
rolling resistance one which stays pretty much a constant at all speeds. For
an average modern streamlined but heavy car like my Focus the drag force is
fairly immaterial below 30 mph. At 45 mph the engine power being used to
overcome drag is about the same as that being used to overcome rolling
resistance. At 65 mph it's twice as much and at 80 mph three times as much.

However pick a light but less streamlined car like an old Fiesta or Golf
from the 80's and the picture changes. At 2000 lbs with driver rather than
the 3000 of the Focus but 15% to 20% higher drag the relationship is very
different.
The power being used to overcome drag is the same as that being used to
overcome RR by 35 mph. At 60 mph it's three times as much and at 80 mph five
times as much.

Light cars therefore benefit from going slower much more than heavy ones in
terms of fuel consumption. This is exacerbated by the fact that the large
engines of large heavy cars, as with all petrol engines, only work
efficiently at reasonable throttle openings and at low speed on a whisper of
throttle the cylinders are not filling well. A small light car with a small
engine is doing much better in all respects here. You want a diesel for good
efficiency at any power output.

What this translates into is my Focus doesn't really get much different fuel
consumption whether I do 60 mph or 80 mph. It seems to peak at about 38 mpg
and I just can't improve on that. However an old Fiesta XR2i I used to run
which gave 34 mpg on average once achieved 51.5 mpg when I stuck to 40 mph
on A roads and 50 mph on motorways during the fuel shortage in 2000. If I
gave the Focus the same treatment I doubt I'd beat 40 mpg by much. It's just
too heavy.

The formula for good fuel economy is very simple. Small, light, aerodynamic
cars with small low friction diesel engines. The 100 mpg practical car is a
very easy thing to design but we'll only see them when fuel prices bite even
harder. I've designed one in basic terms which would take me from London to
Aberdeen, which I do fairly regularly, for 1/3 the fuel cost of the Focus.
At present a round trip costs me £150. That's a big chunk of a holiday cost
when you can fly abroad with Sleasyjet for a tenner. At £50 it wouldn't be
much of a burden.
--
Dave Baker