Putting regular unleaded fuel in a car that requires premium is generally not recommended and can be harmful to the engine over time. While most modern engines have knock sensors that can adjust timing to prevent immediate damage, this compromise comes at a cost. The primary risk is engine knocking, also known as detonation, which occurs when the lower-octane fuel ignites prematurely under pressure. Consistently using regular fuel can lead to reduced performance, lower fuel economy, and potential long-term damage to the engine's internal components, such as the pistons and valves.
The key difference is the octane rating, which measures a fuel's resistance to knocking. Premium fuel (typically 91-93 octane) is engineered for high-compression engines, commonly found in luxury and performance vehicles. These engines are designed to extract maximum power and efficiency from high-octane fuel. When you use regular unleaded (87 octane), the engine's computer (ECU) must retard the ignition timing to prevent knocking. This safety measure directly sacrifices power and thermal efficiency.
Here’s a quick comparison of the potential effects:
| Potential Consequence | Short-Term Use (Single Tank) | Long-Term/Habitual Use |
|---|
| Engine Performance | Slight reduction in horsepower and throttle response. | More significant and noticeable power loss; engine may feel sluggish. |
| Fuel Economy | Potential decrease of 1-3 MPG due to less efficient combustion. | Sustained lower fuel economy, negating any cost savings at the pump. |
| Engine Knocking | Occasional, mild knocking under heavy acceleration (e.g., merging onto a highway). | Frequent, louder knocking; increased risk of damage to engine components. |
| Emissions | Slight increase in tailpipe emissions. | Higher emissions over time, potentially affecting sensors and the catalytic converter. |
| Engine Longevity | Minimal to no risk for one-off use. | Increased risk of premature wear on pistons, rings, and valves. |
If you accidentally fill up with regular, it's not a cause for panic for a single tank. Simply drive gently—avoid hard acceleration and towing heavy loads—until you can refill with the correct premium fuel. However, making a habit of it to save money is a false economy. The potential repair costs from engine damage will far outweigh the few dollars saved per fill-up. Always consult your owner's manual for the manufacturer's definitive recommendation.
