"Most drivers have, in their heads, a rough idea of how many car journeys £20 worth of fuel will get them"

Mmmmm, I have, in my head, a pretty accurate idea of how far a full tank will get me, and I have never, ever, in 43 years of motoring, run out of fuel - even when an old banger (Vauxhall Viva Rotbox) back in 1972/3 had a non-operative fuel gauge for over a year!

I find budgetting for four full fills a month easier than six, or maybe seven, or perhaps eight, or might it be nine, or could it be ten, part-fills! :-)

But maybe, at this early hour on a Saturday, that's just me being a contrary, grumpy old man! LOL!