I have some tidbits of info about this. Using the Premium gas is the best way to go for several reasons. You WILL NOT save by using the lower grades! I did a careful test on this driving across country CA to FL. My gas mileage (including horsepower) was so far improved, that, after doing all the math calculations (no I am not a Nerd) was interesting! You actually get further down the road (more bang for your buck) with the Premium (dollar for dollar) than with the lower grades.
Another fact is that, the lower the octane, the hotter the fuel burns. The Rialta temp gauge (in mine) never really changes, but in my older Ford motorhome the difference in readings was dramatically lower with the better gas. So, you run cooler, more powerful, better for the engine, and get further down the road for the dollars spent. You will save by using the Premium! Also, takes the guilt and stress out of deciding.
Paul