In the US we have usually three different gasoline octane grades to choose from. In the south that’s 87, 89 and 91 octane (this is different from the European number). Modern fuel injected cars can usually consume any of the fuels without experiencing the knocking phenomenon in combustion engines. This is due to the engine computers, that advance or retard ignition timing based on many inputs from the engine. The price per gallon differs about 25-29cents between the lowest and grade and highest grade fuel. So common recommendations say to use any fuel grade you want, unless you put high stress on the engine, like towing a trailer, or driving lots of uphills. While there’s nothing wrong with that, it might be actually cheaper overall to use the more expensive high grade fuel over the lower grade. How’s that? That’s because the mileage of the car deteriorates with the lower grade fuel. It’s enough to make up for the difference in price – at least on a turbo charged vehicle like the Mazda CX-7. Instead of achieving 23 mpg in highway driving we only saw 19 mpg on 87 fuel.

Another interesting observation is that with rising fuel prices (as we’ve experienced them in 2008 when we paid up to 4.25 per gallon) the cost of driving a car that requires 91 octane fuel becomes relatively cheaper than one that consumes 87. That’s because the price delta between 91 and 87 stays usually constant in absolute terms (25c), so percentage wise the gap shrinks with rising gas prices. This led to the interesting scenario, where it was cheaper to drive my more fuel efficient BMW which consumes 91 preferably (21mpg), over the Ford Explorer sipping on 87 (16mpg).