Recently, in spite of increased fuel costs, I’ve seen more and more consumers at the pump using mid-grade or premium gasoline in their cars. One gentleman in particular was filling up his 2005 Honda Civic EX with 91 octane gas. Curious as to why he was using premium fuel in his economy car, I asked him why he chose that product. He told me that he has gotten better gas mileage through using the higher octane gasoline.

This is a common belief with many drivers out there today. In order to figure if the octane rating in gasoline really does improve your miles per gallon (mpg), it’s important to know exactly what it means, and how gasoline burns off in your engine.

In the early 1900’s, gasoline engines compression ratios were restrained due to the unwanted "knock" that would rapidly destroy an engine. "Knocking" is the best description of the sound an engine makes when too low of an octane fuel is used. Restrained to lower compression ratios, the engine is limited significantly as to how much power they could put out. As engineers sought to develop higher power and more efficient motors, they had to find a way to overcome "knock".

"Knock" was commonly believed to come from the pre-ignition of gasoline in the engine, but was later found to arise from a violent pressure rise after ignition. This pressure rise is due to a series of pre-flame reactions in unburnt "end gases", prior to the front flame’s arrival. In order to contain "knock", these pre-flame reactions must be controlled as well.

This is the sole purpose of the octane level of gasoline. Having higher defense above what is necessary against "knock" will not improve your fuel mileage. Lower compression economy motors are made to run on 87 octane and higher compression and turbo-charged cars are generally made to run on 91 octane. As these vehicles are designed to run most efficiently at their given octane and the fact that higher octane fuel is more expensive, any excess is just a waste of money.

The only benefits that higher octane fuels these days provide are the additional additives and cleaning agents some gas companies employ. A cleaner engine will run smoother and more efficiently but is it really worth the average extra twenty cents a gallon? There are other fuel system cleaning services that can be performed at a major service for far less coin over time. Always consult your owner’s manual, local automotive technician or the dealer prior to making any decision when changing fuel type or servicing your fuel system.