a whole lot of crap snipped

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.
Roger, me lad - you wouldn't happen to be a britiah trained engineer,
now, would you?? In what discipline of engineering?