FWIW & IMHO, and none of my business, but could possibly use a more accurate method of checking your mileage: Gas gauges are generally inaccurate at both extremes of the guage, but are nortoriously inaccurate at the lower end of the spectrum.
You might want to try this method, which has always worked for me.
Fill the tank till it shut off about the 10th time, record your mileage, and set the trip odometer.
Do the same after several days of use. As a general rule of thumb, the more the distance, the more the accurate teh reading.
It is important to fill the tank on the second round the same method as the first time. Same pump, same 10 times or whatever you used the first time to determine that the tank is full, car heading in the same direction and approximately the same location. Divide the miles traveled by the gallons taken on, complete to at least the tenth digit. Same approximate temperature also plays a part in the equation, but i do not know how much.
For instance 200 miles traveled and taking on 10 gallons of gas,(filling to the same approxinate depth in the tank) equals 20 MPG.
I am curious how this affects your calculation as above of 3 gallons of gas went about 15 miles= 5 MPG= ouch
