Using a vacuum guage and gear selection towing for best MPG?

You should be more concerned with transmission temperatures than MPG. Gas is cheap, at least the difference between 8.0 and 8.5mpg that you may might be able to save. For naturally aspirated gas engines - one higher gear will use same to slightly less fuel overall, better throttle response, higher average speed, and lower trans temps. You probably want to be in the 2500-3000rpm range.