On Wed, 19 Jul 2017 12:53:25 -0700,
Bob F wrote:

But of course, that fuel economy was based on odometer readings, which
are reading 26% low with the 19" tires. And the acceleration was only
slightly different because only the 1st gear part of the acceleration is
going to be changed much. After that, the gears keep the engine near
it's optimal power range for acceleration. On the highway, the engine
will be operating at 26% lower RPM, so depending on the power curve of
the engine, mileage could go either way.

Even worse, it could be argued that "standard methods" of calculating mpg
are off by a factor of about a mile per gallon either way, which means
that, say, 20mph is anywhere from 19mpg to 21mpg.

Certainly anyone who calculates mpg with decimal places is ignoring sig
figs in their measurement capabilities in that nobody does a reliable test
loop (and even those who do, in Austin Texas for example, get sig figs not
much better than that).

WARNING: I have to look up if challenged the actual reliability but I
"assume" that most people think their measurements are infinitely more
accurate and repeatable than they really are. If it's not as big as +1 and
-1 mpg accuracy, it's not that far off. Certainly there is no decimal place
in typical mpg calculations by a consumer.