15.0 mpg to 21.6 mpg!

Your calling a one time mileage measurement "real world" is like calling Jenna Jameson a virgin schoolgirl.

You cannot measure fuel mileage on e one time run. Fuel mileage is measured using the average of several trips, not just one. You must average out several trips, as there are always variances from trip to trip. Also, use the full tank method as described. Find the mileage of each individual trip, then average them all together for the total average MPG. The more trips you average together for a final average MPG, the more accurate your figure will be.