Is there some kind of gas filler or additive used that makes is seem like you are filling up but you are getting less gasoline?

Answer 1

There are so many variables that might be involved in your calculations regarding 1/2 a tank and 1/8 of a tank, i.e.: Was the air pressure in your tires the same both ways of travel? Did the relative ambient temperature remain constant? Did you purchase the gas at the same vendors during both legs of the trip? Are you certain that your engine did not develop any changes while traveling? Did you stick to the same octane rating on all the gas you purchased? I'm just trying to point out that it could one or many factors. Gas stations are a highly regulated business by many governmental departments- I doubt anyone was tainting their gas.

I meant the made it to my 1st destination from a full tank to 1/8 tank.

All conditions were the same. No change in weather, pressure, etc. I used the same octane as before. I seemed to get better milage from an exxon station but I don't remember where I filled up the first time.

I do remember that the first station was inside the city and the Exxon station was along Interstate 10 in Louisiana and not in town. I was thinking that since people develop habits of going to the same station to fill up that it would benefit them to come back as much as possible rather than a station along the highway where you get fuel as soon as you need it.

Wind direction is the likely answer. I have achieved nearly 4 additional MPG going one way, and lost it on the return trip.