Acura Repair: test gas mileage, lawn mower engines, gas mileage

Question
This is not Acura specific question.
I want to test a product for mpg improvement.
How do you test mileage. I already know the obvious! The shut off
mechanism on nozzle is a variable. I live in Calif, it may be different here.
Also you can't fill to the brim, top off, without losing fuel. I am racking my
brain. Your suggestions for accuracy. I am willing to rent a car. run a car to
empty, lawn mower engines, and or dynometer!!
Thank you kindly

Answer
hmmm... by product, i'm assuming this isn't a car?
i would have to say have just enough gas in it to run it til it is fully warmed up. let it die.
use a precise measuring cup of some sort. a kitchen cup. a certain gas can filled to the top. anything you can come up with so no matter how much gas you add, its the same amount. you can interpolate the full number from that. even use the same funnel every time. on a small engine getting the mileage on a warm engine means more.
and make sure to run it on level ground.

if it is something that you have to fill at a gas station and cant let it run til it dies, drive until the low fuel light comes on. refill with the same pump at the same station every time, allowing it to turn itself off. run until fuel light comes on again.

you aren't looking for perfect accuracy, you are looking for consistent inaccuracy.

John