My car “recommends” premium gas, but “allows” anything as low as 87 octane. I originally asked the consequences of this here somewhere, but did the experimenting myself. I used the onboard computer for determining mileage:
Premium (91 octane): 22.5 MPG average
Regular (87 octane): 20.5 MPG average
Let’s say the price difference is $0.20 cents (although I often see a larger difference!). Twenty gallons of premium gas cost $4.00 more than regular unleaded. So, Twenty gallons of regular get me 410 miles, while twenty gallons of premium get me 450 miles. The difference is 40 miles, which would have taken 1.95 gallons MORE of the cheap gas. If unleaded cost $2.05, it would be a break-even prospect. If unleaded costs less than that, then the premium is a waste of money. If the difference in premium is more that $0.20, then it’s an even bigger waste of money.