Well, prepare to be shocked, or at least I am.
Using my 4 month new ES 330 in combination highway and city driving,
Using Regular gas I drove 721.7 mi and used 36.26 gallons for an average of 19.90 mpg, at price $1.76 per gallon. It comes to 8.844 cents per mile.
Using Hi Test gas I drove 850.9 mi and used 41.02 gallons for an average of 20.7 mpg, at price $1.87 per gallon. It comes to 9.033 cents per mile.
For a trip of 1,000 miles, the difference in cost between the two comes to $1.89.
Tell me you expected it please. I didn't. I thought Regular gas would come out way ahead in cost savings "in the long run". Well it doesn't!
I am an Engineer and an environmental/efficiency geek.