15.0 mpg to 21.6 mpg!

Overall, I think the work is sound, the scientific method is in full effect, the statistical analysis just needs a few more data points.
Would be a shame to waste such good effort on a change in density due to fuel temperature or atmospheric conditions (so more trips certainly wouldn't hurt), but the OP is definitely putting in more effort than most when it comes to tuning, and the results are definitely in the right direction.

Don't beat the guy up because one data point MIGHT be an outlier.