I did, in fact, do an airflow test before I placed the VGs. The car is very dusty and I ran my ever-exhuberant washer squirters overtime to get a flow track all the way past where I placed the things. The track was very obvious. I would have done an oil drop test if it would come off as easy as washer fluid tracks. The paint is a flat paint and everything soaks in.
I agree that extended runs would be better, statistically speaking. But I believe the difference would be negligible. The run was completely highway miles, and two 100 mile trips that were that close is fairly indicative of true mileage, IMO. I don't much care to worry about city mileage. Some times I get better economy, sometimes I just have to mash the gas and listen to the engine sing and go real fast. I idle a lot, too.
My margin of error appears to be about .4mpg-ish. I'm not going to take great pains to get that down to .1. I will, however, make amendments should anything change appreciably.
I don't do long drives any more that have a real purpose, so Sat/Sun a.m. drives that last a cigar or two will have to do.
I think you are so willing to believe that your modifications help that your driving improved subconsciously, which is what really improved the mpg and also is what I suspect was the case in my early days, no harm done thou, that's for sure.
A FE gauge should be standard equipment in every vehicle.