How I Improved My Gas Mileage

While my 5 year old son was washing our '03 Suburban, he decided to fill up the gas tank with the garden hose. Unfortunately, he forgot to mention this to my wife as she drove off to run errands. She was able to get about 4 miles from the house before the burb gave up. I was out of town, so a tow truck brought it home where it sat for a few days.
Prior to this point, we were getting an average of 15 miles per gallon. Sometimes less.
I wasn't exactly sure how much water had been pumped into the gas tank. I took took the fuel line off the fuel rail and used the fuel pump to pump the water into some 5 gallon buckets. The first bucket was completely full of water, the second and third buckets were about half full of water. It came to about 10 gallons of water. Once I couldn't pump anything else out of the tank, I put 5 gallons of isopropyl alcohol into the tank, and then pumped that out. I put 5 more gallons of the alcohol in the tank and started the engine. I drove it to the gas station and filled the tank. Now we get 17 mpg in town and 20 mpg highway. I haven't figured out the physics of it yet, but I think the computer recalibrated for the alcohol, or the alcohol and water cleaned out the combustion chamber. I wouldn't recommend this method for better gas mileage, but what else could I do? (Short of dropping the tank!)