On Sat, 22 Jul 2017 19:29:49 -0700 (PDT),
trader_4 wrote:

1 - Fill the tank once, drive until it's near empty, fill it again.
Your accuracy is greatly affected by your ability or inability
to fill it to exactly the same level. If you're off by a gallon
on a 15 gallon tank, it;s 7%.

2 - Fill it at the beginning, drive it a much longer distance,
through 10 tanks worth of gas where you have the pump reading
on all of those, then fill it the last time to as close to the
original fill as possible.

Method 2 reduces the inaccuracy due to not filling it to exactly
the same level by an order of magnitude. If you're off by a gallon
between the first and last fill, it's an error of ~0.7%.
You still have whatever the accuracy of the pumps are to deal with,
but I agree I'd trust that the pumps are going to be a lot
closer to the 0.7% accuracy than 7%. I'd think they are better
than 0.7%. And like you said, in between
the first and last fill, it doesn't matter if you fill it all the
way or only half full, etc.

I agree that averaging the "fill level" estimate is a great way to reduce
that huge error of guessing where the last fill level was.

I never disagreed with that, although I may not have realized it in the
very beginning. So, we have to assume a 10-tank fill when we state what our
innacuracies are.

I'm ok with assuming a 10-tank fill.

But remember, your inaccuracy is no better than your worst measurement, so,
what do we do about a tripmeter that has no decimal point?

I will wager that most people use the tripmeter and not the odometer.

If the tripmeter/pumpmeter has no decimal point in the numerator, you can't
possibly get a decimal point in the resulting calculation.

That's how math works.
Isn't it?

So it's not "me" you'd argue against.
It's math you have to argue against, since I'm just the messenger.