"dennis@home" gurgled happily, sounding
much like they were saying:

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.

So why do large commercials use lots of fuel at a constant 30, more at a
constant 40 and even more at a constant 50? If drag isn't the cause what
is?

I'm not a truck driver, so I don't know if that's the case or not. If it
was, I'm sure the haulage firms would be restricting their driver's
speeds - yet they aren't.