Dear Ole Mechanic;Grades of Gasoline

How can I tell which grade of gasoline I should use in my car?  The claims for super-premium versus mid-grade versus regular are very confusing.  Please help.

Thank you.    


Dear M.S.;

The answer may seem simple, but--as usual with cars and trucks--there can be exceptions.  The simple part of the answer is that the proper grade of gasoline is given in the owners manual for your car.  It is usually given as an octane rating, and for most cars and trucks, it is 86 octane--unleaded regular gas.  The exceptions to that occur in high performance vehicles like a Chevrolet Corvette or the F 150 Ford Harley Davidson special pickup which has a supercharged engine.  The owners manual for these vehicles calls for a 93 or higher octane gas.  Gasoline with that high of an octane rating is called super-premium or sometimes just premium and it is not what the majority of cars and trucks need.  Again, the recommendation for a particular make and model will be given in the owners manual, but it is only a recommendation.  Your car probably has regular or 86 octane recommended, bu--as we all know--there are many differences between supposedly identical cars as they come from the factory.  One will seem to ride better, another seems to have more get up and go, yet another ‘identical’ car will get better gas mileage, and--of course--there is an occasional lemon.  So, identical cars are not identical, but they do come with identical owners manuals. 

With that in mind, I’ll go back to your question: “Which grade of gas should I use?”  Start with the grade that is recommended for your car, and check the fuel mileage very carefully over several tankfulls of gas.  Then run at least two or even three consecutive tankfulls of the next higher grade of gas, while checking the fuel mileage carefully.  If the higher grade of gas improves the fuel mileage, you will probably notice an increase in performance also.  An improvement in mileage and performance indicates that your car should have the higher grade of gasoline.  If there is no increase or even a decrease in fuel mileage, then the lower grade of gas is all that your car requires.  A word about octane is in order here.  Higher octane gasolines are made that way to prevent spark knock or "ping"  Some people say that spark knock or "ping" sounds like a bunch of marbles rattling around in the engine.  Others say it sounds like a diesel engine knocking at idle.  The higher octane gas is made that way at the oil refineries by mixing in more of the lighter, less powerful--but spark knock resistance--parts of crude oil.  In short, your car should have the best power and fuel mileage on the lowest-octane gas that it can burn without having spark knock.  The problem with listening for the knock or "ping" is that today's cars and trucks have electronic fuel-injection systems which have electronic sensors that can detect knock or "ping" before we can hear it.  The electronics then retard the ignition timing to stop the knock or "ping" and keep it from being heard.  This allows almost all engines to run on regular, but the retarded ignition timing will hurt fuel economy and power.  Trying a higher grade of gas which has more resistance to knock or "ping" and then checking the fuel mileage is about the only way to tell is your car requires the higher grade of gas. 

Most higher grades of gas--which are more expensive--have more additives and can help keep today’s fuel systems and injectors cleaner and running longer.  Of course, cleaners can be purchased separately and occasionally added to your normal grade of gas .  What does your Herr Professor do?  Once or twice a year, I will run a tank full of very expensive super premium through my vehicles and the rest of the time they get regular from any known-brand gas station.

One thing to watch out for is a sudden drop in fuel mileage and poor running and performance.  It was documented in California a few years ago that a less than honest gas station operator purchased 1000 gallons of 50-cent a gallon heating oil and mixed it with 5000 gallons of $1.25 regular gas.  By mixing it, he was able to sell his heating oil as regular gas and made 75 cents a gallon profit.  The problems of poor fuel mileage, poor performance, and plugged up fuel filters did not uncover the illegal action.  An inspector checking papers was the only way the illegal activity was found.  Should you experience a sudden drop in fuel mileage and poor performance without the "Check Engine" light coming on, try a different brand of gas before heading to your mechanic.

I have not touched upon the "leaded versus. unleaded gas" topic,  because not too many leaded-gas vehicles are still running around; however, if the question is asked of Herr Professor, I will be happy to go there.

Herr Professor Nuzanbolts