On Thu, 20 Jul 2017 17:32:39 +0100,
Mike Coon wrote:

What is a realistic accuracy & precision of typical MPG measurements when
measured by the consumer using the typical method of dividing their
tripmeter miles by the gas-pump gallons during fillup?

Trip meter miles depends on circumference of driving wheels. I know my
speedo closely matches readings of roadside radar displays or my GPS, so
I guess trip meter miles will be accurate too.

Every reading a mom and pop does has inaccuracies that, I posit, are
tremendously higher than most people seem to think they are (at least most
people who quote mpg figures with decimal places in them).

Most people have a tripmeter reading and a gas pumpmeter reading.
Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or
repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but
basic logic dictates that the errors compound such that there is likely
(IMHO) no way to get anywhere near decimal-point accuracy, and worse,
probably plus or minus 1 mpg is the closest anyone can get in terms of
repeatability and precision.

Even the EPA's $360,000 machine only claims plus or minus 2% of the
indicared reading. I can't find where I got the notion that a mom and pop
can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
calculation - but I'm still seeking those numbers as we speak.