> I was just thinking about gas milage for some reason. =A0...

We used to use "per acre" for fun.

As you'll be aware, mpg is distance per volume so has units of area^-1
while litres/100km (now in common use here) has units of area.

One consideration for visualisation is that the diameter of a circular
plot varies with the square root of area so eg 50% more consumption
only produces 22% increase in diameter. Consistent with this reduction
in apparent increase - I have found that people's estimates of the
ratios of two circles is usually low, and the increased area in the
added peripheral ring is less intuitively obvious.

So - a linear graph with units of area may better serve the purpose.

However, as this is an aid only, and as a square root law display was
potentially possible, you could arbitrarily introduce some other law -
eg say square law, so if you doubled consumption you'd get 4 times the
height or length of display. If the display is in terms of maximum
economy (mpg style) and the maximum value only slightly exceeds the
best possible economy in practice, then the display will (hopefully)
be towards the high end often when driving economically, but will
reduce rapidly as you get more heavy footed.

Parallel though train: I'd long thought that the vacuum gauge of yore,
used as an economy aid, could be replaced by a true economy gauge.  I
hadn't thought about making it non linear. Modern electronics make the
on the fly calculation of true fuel economy (aka mpg) trivial, whereas
when vacuum gauges were all the rage it would have been extremely hard
to make a true instantaneous mpg meter. (A relatively simple
mechanical analog computer would probably have been the easiest and
probably best solution).


   Russell

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