On Wed, 19 Jul 2017 22:59:45 -0400,
wrote:

So 20mpg is really from 19mpg to 21mpg, which makes calculation difficult
(when I was looking up the wheel size stuff earlier today, Consumer Reports
said as much so that's why I'm curious how close you think you can get to a
repeatably precise figure (yes, both repeatable, and precise).
With the fuel mileage rig I had (for carbureted vehicles) you could
see the differnce made by changing tire pressure by a few PSI if the
wind didn.t change. You could see the difference from winding down a
window.

Take everything I say with a grain of salt because I'd have to dig up the
references of where I got this idea that even the EPA can't get better than
4% (as I recall), and they don't measure volume (since that's inaccurate).
They measure weight. And they used, as I recall, plastic bags filled with a
known *weight* of fuel.

In addition, they ran on test course, and they repeated the tests.
And even then, they couldn't get better than 4% as I recall.

Unless I dig up the reference, that's all hearsay, but, as I recall, nobody
is going to get accuracies and repeatabilities anywhere near decimal places
at home if they're not repeating the tests under extremely well controlled
circumstances and weighing the fuel consumed.