So the 1999 Accord's owners manual recommends 87 octane or better for my V6, I decided to experiment with different grades just to see if 87 is truly the best fuel for my car, or if the added cost of premium would do anything at all for the performance or economy of the car. Over 6500 miles worth of gasoline data, I've averaged approximately 22 miles per gallon. One experiment I ran throughout the winter/spring was to alternate the type of fuel I filled the car with. Because modern ECU's can take an entire tank of a higher octane level before adjusting how much fuel is pumped into the cylinders, I put in two tanks of regular unleaded, and then switched to two tanks of premium unleaded. I continued this pattern from January straight through until about two weeks ago.
Based on the data, I average 33 miles additional per tank when I fill the car with premium unleaded instead of regular unleaded in pure city driving, and 53 miles per tank more on mostly highway driving, or in my 16 gallon fuel tank, my car averages slightly more than 2 mpg better in city driving with premium unleaded than regular unleaded, and slightly more than 3 mpg better on the highway with premium. What does that mean? That means that the old rule of saving money by putting in regular unleaded doesn't really work for my car...however, it's entirely dependent on the cost of gasoline.
Now, here's where the tricky math comes in. To fill my car with premium unleaded instead of regular unleaded costs approximately $.25 extra per gallon, or about $4.00 per tank. At what point does it make more sense for me to buy premium over regular. At $4.00 per gallon, for example, that extra $4.00 actually saves me about $6.28. The break even point for in town driving is at $2.55 per gallon. On the highway, the break even point is significantly lower...about $1.66 per gallon.
I wondered after doing the math on this why Honda wouldn't recommend premium to fill this car, so I went to find historic gas price data. In 1998/1999 when this car was manufactured, the price of gasoline in the US was only $1.29 in today's inflation adjusted dollars. What that means is it was even less expensive then. That price is well below the $1.66 per gallon that is my break even point for highway driving. Granted, gas hit that price point in 2000, but it didn't hit the $2.55 per gallon until 2007, and was below it again by 2009. Quite frankly, there was no reason to recommend a higher grade as when the car was built, no one was even predicting that gas would go above the break even points.
After watching what happened to the price of gas this summer, it's improbable that I'll ever get to the point again where regular unleaded is less expensive to operate in my vehicle than premium unleaded. Try it for a few months in your car and see what your results are. I'd be interested to see if other people in different cars have the same result I did.