On Thu, 20 Jul 2017 11:59:40 -0000 (UTC), Mad Roger
wrote:

On Wed, 19 Jul 2017 23:07:03 -0600,
rbowman wrote:

With my scanguage calibrated to my GPS, over a 2 or 3 tank run I can
be accurate to tenths

Mad Roger lives in the past. What I find cute with the scangauge set for
instantaneous reading is when you get off the throttle, the injectors
shut off, and the reading goes to 99.999 mpg.

Remember I said 4% accuracy, so let's see if my statement holds water over
the next few days. If that is correct, then anyone quoting accuracy with a
decimal place in it is ridiculously misled.

Doing a quick search, Car & Driver says the EPA gets to 1% with what
appears to be a 350 million dollars investment in tools.

"Measuring fuel economy during the tests is likewise hugely complex, which
is why the automakers and the EPA both follow precisely the same protocol.
For openers, the chemical composition of fuel varies slightly, so simply
retrieving it from a local gas station won¡¦t produce repeatable results.
The EPA has a specialized company manufacture small batches of consistent
fuel, which is 93 octane ... Before being used, the gas is analyzed to
measure its properties, and fuel economy is then calculated based on the
measured carbon content of the various tailpipe emissions¡Xunburned
hydrocarbons, carbon dioxide (CO2), carbon monoxide (CO), methane (CH4),
and oxides of nitrogen (NOx)¡Xthat are collected in bags made of a special
Kynar plastic. A $350,000 gas-analyzing machine then makes minute
measurements. The one-percent accuracy of this machine from Japanese
company Horiba is amazing considering the minuscule amounts of some of the
exhaust constituents¡Xsome in quantities as low as a half-dozen parts per
million.
http://www.caranddriver.com/features...economy-page-2

So what's 1% to a typical consumer?

Reading the whole article, I can't tell if that 1% is 1% of 20mpg or 1% of
18 gallons.

If it's 1% of, say, 20 mpg, then that could mean it's +/- 0.2 mpg (or 19.8
mpg to 20.2 mpg).
If it's 1% of, say, 20 gallons, then that could mean it's +/- 0.2 gallons
(or 19.8 gallons to 20.2 gallons).
The problem is getting an accurate reesult over varying conditions.
Getting extremely accurate results in a fixed condition is simple.
Same road, same speed, same weather conditions, repeatability is
excellent and simple equipment can be very accurate. For instance,
calibrated fuel supply, running start on measured distance at
predeteermined speeds - make modification and retest.