Dayton Daily News

Wheels: Ralph of Springfield writes, “Will it hurt my car if I use regular-grade gasoline instead of high-test gasoline? I own a V-6 and it does not ping or make noise going up a hill.”

Halderman: Most vehicles today are designed to operate on regular (87 octane) gasoline unless otherwise indicated in the owner’s manual. Some examples of vehicles that often require premium gasoline (91-plus octane) include supercharged or turbocharged engines and some high-performance vehicles. Even when the manufacturer specifies that premium grade should be used, regular or mid-grade (89 octane) can often be used without harm to the engine. No harm is done because most engines are equipped with a knock sensor that detects engine damaging spark knock, also called detonation or pinging. The knock sensor signal is then sent to the engine computer, which retards the ignition timing to reduce or eliminate the spark knock. As a result of this retarded timing, the engine will likely produce lower-than-normal power and reduced fuel economy.

NOTE: While using regular-grade gasoline instead of the recommended premium-grade gasoline could reduce engine power, using premium-grade gasoline in a vehicle designed for a regular grade of gasoline will not increase the power output.

Wheels: What does the (R + M) divided by 2 mean?

Halderman: The octane rating posted on the pump is the average of the two methods used to determine the octane. The two methods are the Research (R) and Motor (M) method. The research method is less demanding and the rating is higher compared to the motor method.

NOTE: Some European vehicles, such as Mercedes and BMW, indicate that the fuel needs to be 91 octane. This rating is often the research number and is equal to a rating of 87 (regular) using the (R + M) divided by 2 method.

Wheels: What if you do not know what gasoline should be used?

Halderman: Most vehicles require regular 87 octane gasoline, so I suggest that this be used unless one of the following is detected:

1. A label on the fuel gauge in the dash or on the filler door states that premium-grade gasoline be used.

2. There is a noise heard under the hood while accelerating that sounds like loose marbles in a coffee can or clacking valves.

Some vehicle manufacturers “suggest” the use of premium-grade fuel and state in the owner’s manual that regular or mid-grade (plus) can be used, but with a loss of power and fuel economy.

James D. Halderman is an ASE-certified master technician, a member of the Society of Automotive Engineers and the author of 12 textbooks. Submit your automotive care questions to Halderman via e-mail at jhalderman2@juno.com, or write him in care of: Wheels, Marketing Publications Department, Dayton Daily News, 45 S. Ludlow St., Dayton OH 45402.