The Silver Streaker said:
My dim & distant memory is telling me that the 102MPG record set around the coast of Britain was at an average speed of 22mph (presumably mostly at 25mph in 5th).
Not sure what record you're referring to, although it reminded me that VW recently was in the news for some record gas mileage of their Lupo TDI. Here's a short article about it
where it lists the total distance as 4683 kilometers, fuel economy of 2.78l/100km, and average speed of 81km/h.
For those of us used to standard units, that works out to 2903 miles, 84 US miles per gallon, and 50 miles per hour. That's actually pretty dang impressive for a non-hybrid vehicle. It should certainly be possible to achieve a similar feat in an Insight, but 84mpg is quite high, especially for an average speed of 50mph, assuming they mean that in the sense that it includes speed during stop and go traffic, not just cruising on the highway at 50mph.
Anyway, on a similar note, I've found several resources which indicate that the power required to overcome aerodynamic drag increases at the cube of velocity. Now what's the top speed of an Insight? 112mph or so? And by the time you reach that speed, you're probably operating almost entirely on motor power, not so much battery assist, so we'll say that it takes roughly 70hp to get an Insight to do 112mph. Applied to lower speeds, it would thus take the following power levels to maintain speed:
So you can see that it takes virtually no power to overcome aero drag at 30mph, and only a small amount to overcome drag up to around 60mph or so. From there the penalty increases pretty rapidly. This of course typically begs the question as to why you don't get 300 miles to the gallon if you cruise at 30mph. The answer is that motors are not very efficient at extremely light throttle applications and the pumping losses, internal friction, and power to overcome the friction of the transmission and tires is greather than the power needed to overcome aero drag at such low speeds.