scott ) gurgled happily, sounding much like they were
saying :

I don't often drive through "fluid",

Really? Mastered the art of driving through solids or in a vacuum
have you? ;-)

How hard IS it raining where you are? It's quite sunny here.

A fairer example would be cycling.

Cycle ten miles, on the flat, at a certain pedal cadence (engine rpm)
in a low gear.

Now cycle ten miles, on the flat, at the same pedal cadence (engine
rpm) in a higher but still comfortable gear.

Yes, if I go fast cycling I get hot and knackered.

As I said, in a *comfortable* gear.

On my bike I can get to about 20mph for a few minutes at a time, if I
drop that to 15mph I can go for *much* longer.

So let's assume a very low gear and walking speed, and 10-15mph.

If you're cruising (low throttle opening) in a car in a highish gear,
at low revs, that's bound to emit less pollution for a given journey
than similar revs in a lower gear at lower speed - because those revs
are being used for far less time.

Yes, but you'll be using less petrol. A lower speed = less power from
the engine. This = less pollution.

But the engine's turning at the same speed for both. I explicitly said
that. Yes, there's a certain amount more load in the higher gear, but I
also explicitly stated that we weren't talking about a high load
situation akin to your 20mph on your bike.

How do you work that one out? If I'm using more power to go faster,
surely I need to be using more petrol? As air resistance increases
with speed squared, the amount of petrol used goes up quite quickly
once you get to higher speeds.

Which is cancelled out by the higher efficiency of being in a higher
gear. Obviously, there's a point where that's not true, but almost any
car will sit at 40mph or so with virtually no throttle. Try it.