The only way you're going to get an accurate assesment of your fuel mileage would be to fill the tank all the way up at your favorite station. Fill it until the pump shuts off, then go to the next $.25 or so. Record the mileage on your odometer.
Then drive enough to use up a good portion of the tankful, not just 4 gallons. Then return to your favorite station and fill up again using the same exact technique at the SAME pump. This way your car is on the same exact level ground in the same exact spot it was on before. This will greatly affect the way the tank fills. Being on a different level at a different station might cause the car to take one less gallon at one station compared to another. Record the number of gallons and the mileage.
Only then can you calculate a somewhat accurate fuel mileage number. Going to different gas stations and getting only about 4 gallons with an inaccurate odometer won't get you a number that's 'real-world'.
Using the method above (same pump, same station, accurate odometer) and repeating it on about 5 to 10 fill-ups will give you a good number.
Now please try again. No way did your car get 28 MPG.
