Excerpted from You’re Doing It Wrong
This is one that seems logical, but most have it wrong. I did.
Premium gasoline costs more per gallon. More expensive must mean better. Put premium in any car and the engine will run cleaner, more powerfully, and more efficiently than it would on regular. Some owners fill with premium as a matter of pride or as preventive care, reasoning that if it’s better for the cars that require it, it must be better for all cars.
Premium fuel provides no benefit — and in some cases produces slightly worse performance — in engines not designed to use it.
The difference between regular (87 octane) and premium (91–93 octane) is not energy content. Higher-octane fuel does not contain more energy per gallon. The difference is resistance to pre-ignition, also called knock or detonation — the tendency of the fuel-air mixture to ignite prematurely under compression before the spark plug fires. High-compression engines, turbocharged engines, and high-performance engines generate more heat and pressure in the combustion chamber, creating conditions where regular fuel can detonate prematurely, causing the characteristic engine knock and reducing efficiency and power. Premium fuel’s higher octane rating resists this pre-ignition, allowing the engine to run at the compression ratio and timing advance it was designed for.
Engines designed for regular gasoline run at lower compression ratios specifically calibrated for 87 octane. Their engine management systems are tuned for regular fuel. Put premium in a regular engine and the additional knock resistance provides no benefit because the conditions that would cause knock in regular fuel are not present. The engine runs on the fuel the same way it runs on regular — because it was designed to run on regular. The extra cost is pure waste.
Some modern engines with knock sensors can adjust timing to take modest advantage of higher-octane fuel even when not specifically designed for it, producing a small measurable gain in power output. The gain is typically minor and does not offset the cost premium at the pump over any reasonable driving period.
Read the owner’s manual. If it says “regular unleaded,” use regular unleaded. If it says “premium required,” use premium — the engine genuinely needs it and running regular will produce knock, reduced power, and over time potential engine damage. If it says “premium recommended,” regular is acceptable but premium will produce the performance the engine was optimized for.
The fuel grade recommendation in the owner’s manual is an engineering specification, not a suggestion.
Excerpted from You’re Doing It Wrong
