All kidding aside, it is disingenuous to you, and over-estimating mpg to count "every drop of fuel" on a single fill-up (to the 'thousandths of a gallon' - really?). You can do "instantaneous math" like that - but it will be inconsistent.
Some fill-ups, those in which the hand "lingers" in pulling the gas pump lever, you spill a tiny bit, your "attention to detail" wanders, hot tank-cold fuel, cold tank-warm fuel, expansion and contraction, will equate to instantaneous results that are skewed from previous individual fill-ups.
The lack of consistency may find you asking, "Is there something wrong with my scooter? One day it's 90 mpg. Another day, 82 mpg." I will say, "Nothing is wrong!"
To reduce or eliminate instantaneous error,
keep a log at each fill up of
1.) the odometer reading and
2.) fuel added to the tank to one one-hundredth (0.01) of a gallon.
Cumulative mileage is current mileage at fill up minus Log Day 1 mileage
Cumulative fuel is EVERY gallon, to 0.01 of a gallon, through the scooter since Log Day 1
AVERAGE miles per gallon is cumulative miles ridden divided by cumulative fuel.
Today ==> 89 mpg instantaneous
In a year ==> (say) 78 mpg AVERAGE, and only then ask, "Is there something wrong?"
