Excellent experiment.
Your class of vehicle has a lot more to gain, in terms of total fuel saved, than the small hondas, toyotas, etc.
I work at a Wal-Mart Vision center. Wal-Mart changes their signage about twice a year. Guess what most of the signage is made of...coroplast.
If you ask nice, you might get multiple shopping carts full of big sheets of coroplast in almost pristine condition. It might have a picture of a handsome man modeling some handsome jeans on it.
Premium vs regular
today is a much different question than premium vs regular from 5 or 6 years ago because of the increased use of ethanol. Premium may very well produce higher fuel economy IF it has less ethanol than the regular. It is rumored to be so in some places. There are several ways to test it without too much time, effort or money. A google search for "ethanol test for gasoline" will get a lot of worthwhile hits. Here's a youtube vid for those who want to try one:
It's just a calibrated test tube. You put a certain amount of water in, add gas up to the line and see how much the aqeous phase goes up. Most of the ethanol will migrate out of the gas into the water. They are about fifteen bucks.
Please carry on.
troy