Tom Quackenbush
There would be an expected reduction in gas mileage as retarding the timing and-or enriching mixture results in extracting less energy per pound of fuel. But the difference is so negligable it would not offset the difference in cost even between regular and midgrade.
In fact, the first few tanks we ran in the new car (I guess I haven't mentioned it's a Chrysler Pacifica with a 3.5L V6 rated at 250HP) were 89 octane and the mileage computer was showing disappointing results - the highway mileage was coming in well below the EPA estimate. Though it's been getting 87 octane, the average mileage per tank has actually come up a little bit, but I wouldn't attribute that to the octane rating of the fuel. This is the first car I have owned that didn't get EPA mileage out of the box.
Going back to extracting energy from gas. Many people think that there is more "power" in higher octane gas. Not true. In fact 87 has slightly more energy available per pound than 91. But that difference is very slight. The knock resitance of premium gas allows techniques like higher compression and advanced timing to extract more energy from the fuel.
Diesel fuel has substantially more available energy per pound than gasoline. Yet if diesel was rated using octane numbers, it would usually be somewhere in the 50s.
