I think it certainly is the "right" time to offer something like this. We are awash in self help books, so why not a short course on driving technique to improve fuel mileage.
What do you think people would pay for something like this?
The only problem I see is your location. I've got relatives in Texas, and I didn't think you where able to tell a Texan how to do anything. They're pretty headstrong.
Hope it goes well.