^ to calculate gas milage you need to know how much gas you put in the car. 2 bars means nothing. The meter on the car isn't perfect, if you on any incline etc forgetaboutit. To get your gas mileage fill reset the trip when you pull up and fill the tank to campacity then read the number of gallons you put in it. miles/gallons=mpg.

All of this of for nothing because your not going to prove anything with the test you are doing anyway. Too many variables. Humidity, AC, weight in the car, heck your own body weight fluctuations, driving conditions, stops, shifting patterns, speed, etc... You could get as much as a 40 -50+ miles difference.

Now to add to the fact that your experiment is flawed, just because on gas stations gas is better one day doesn't mean that it'll be the same tomorrow. Again to many variables. Now to add yet again to all of that, Gas mileage has nothing to do with how good a gas is for you car. Damaging gas could get you better milage but more detonation while the opposite might be true as well. It would depend on what made the gas bad and how your ecu adjusts for it.

Tire pressures never remain constant. WHile you heated up the tires (unless you have nitrogen) the gas expanded and when they cooled down the as condenced. How much to what degree and does it even matter is another story all together.

To do this perfectly you'd have to put the car in a very controled enviorment with a constant load for each tank over and over again.

Basically your experiment has too many flaws to predict if any gas is better or worse unless that gas blew you car up.

 

I never said it was going to be perfect. That's why I called it observations.

 

The only thing I have ever heard is that Shell gas is bad. No basis for this just one of those things I hear from people I know.