15.0 mpg to 21.6 mpg!

And I'm saying you could loose a 1/4 gallon at the pump from one fill up to the next, and in that short of distance it would bump the mpg up 3 or 4. Unless you ran your fuel line to a 2 gallon can that you can see exactly when it's full from on trip vs the other trip, then a 30 trip is almost worthless. How do you know the nozzle clicked at the exact same amount of fuel? Even if you clicked it till it came out, you could still vary from fill up to fill up, and that's huge on a 30 mile trip. That's why I said at least 250 miles so that 1/4 to 1/3 gallon from tank to tank won't have near the impact and will get you very close. That's why Rustyratrod said a few tanks for more accuracy.
Nobody is saying you didn't gain mpg, just saying a 30 mile trip is dust in the wind