>And then if I'm doing 10mph (16km/hr) less than the speed limit on a
>road and have a good tailback of cars behind me, then I'm saving the
>country even more gas as I'm reducing their consumption also!

To which my response is - not necessarily.

My Ford Focus is geared such that I find I have to be in a gear lower than I 
would like to travel around 30 and 40 mph areas. I am not sure if it is just 
the differential/gear ratios selected, or the engine power curve (turbo 
diesel), but unfortunately it does need to be above 2000 rpm to feel that it 
is far enough up the power curve.

So if I come up behind you, and need to change down a gear to stay on the 
power curve, I doubt you are saving me fuel. 

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