Subaru Repair: measuring gasoline added at fillup, for MPG check, fuel recovery, weedwhacker

Question
QUESTION: I am hoping to test a mileage improving catalyst.
I think I have heard most of the stuff ( variables ) in accurately measuring gas
amount used.
To me, the biggest issue ( wind is a tough one too ) is the unreliable manner
in which the gas pump nozzles click off.

I do not want to takes months of trips to measure this products' efficacy.

I propose a few scenarios

10 fifty mile round trips, 5 before adding product, and 5 after adding it.

as opposed to the usual driving till nearly empty ( supposedly, because, my
empty light seems unreliable as well ) then filling up, and so on.

Another idea is running a weedwhacker till it stops, and refill.

This is turning out to be more difficult then I thought.

I also thought about very slowly topping off ( I am in California ) for the sake
of accuracy, but I understand some fuel recovery system can lose excess fuel
too!!

Please help if you can!! Thanks

Desperate for a solution

ANSWER: when performing tests such as the ones you are about to the easiest thing to do is to fill a 5 or 1 gallon gas can and actually measure the amount of fuel you are putting in it .The tests are best done with a trunk mounted fuel cell for accuracy

---------- FOLLOW-UP ----------

QUESTION: Thank you but could you provide a little detail.
"1 or 5 gallon gas can" and do what, idle. I am not clear about your suggestion

Thank you

Answer
What i am suggesting and how we have done some tests here .Is the following
1.Fill the car up with fuel with a gas can, as You can control the level in the neck going down to the tank better than you can with a nozzle at a pump.Use a flashlight to shine down the neck and fill it up until you can just barely see it
2.drive it as many miles as you are comfortable with ,without running it out of gas
3.fill a 5 gallon gas can up with fuel put it in the car do this until you have put the level of the fuel back to its previous level that you set at the first fill up.
By using a combination of 5 and one gallon cans you will know the precise amount of fuel you put in the car and that amount will be the precise amount that you went through in between fill ups.
4.You could fill up at the station and then try and control the final level with a small gas can as well and record the amount of fuel used at the station and out of the can as well but do you really trust the pumps accuracy would be the question there.

Also do not overfill the vehicle ,as that is not what i am suggesting as you could damage your Evaporative system.

Hope that helps

Justin
All Wheel Drive Auto