Each and every automobile out there has been designed to perform optimally using a specific grade of fuel. It’s a pendulum that typically only swings one way: put in higher-grade gasoline than is required and you’re unlikely to see a jump in power of efficiency, but go lower than the manual asks for and you may indeed feel the pinch of reduced output and a thirstier engine.

If you accidentally chose the wrong grade of gasoline at the fuel pump, don’t panic. The consequences are unlikely to lead to any lasting damage to your car or truck – as long as you follow a few tips.

What Is Octane?

The numbers that you see on the fuel pump – 87, 89, 91, 94 – represent the ‘octane rating’ of that specific blend of gasoline, and in turn are linked to its ‘grade’ of regular, plus/mid-grade, and premium. A higher number does not mean that that certain blend of gas is ‘better’ than another one with a lower number – a fact that is muddied by the use of words like ‘premium’. The descriptions of gasoline blends merely indicate how well the gasoline resists something called pre-detonation, or ‘knock’, in an engine.

What is knock? It’s the term used to describe gas igniting inside your engine’s cylinders before the spark plug has fired, and it typically occurs due to the heat and pressure in the cylinder itself. The higher the octane rating, the better the gasoline blend is at resisting this early detonation. Early detonation is detrimental to generating power and can, in some extreme cases, damage the motor.

What If You Make The Wrong Choice at the Pump?

Some cars require higher octane fuel because their engines feature a high compression design, or they use a turbocharger or a supercharger – all three of which can create conditions inside a motor that will lead to pre-detonation of lower-octane gasoline.

Fortunately, modern vehicles feature adaptive ECUs, or engine control units, that are capable of recognizing what type of gasoline is in the tank by observing how it behaves once it’s been injected into the cylinders. A knock sensor listens hard for signs that detonation could or has already occurred, and when detected, the ECU dials back the engine’s timing. In the case of turbo or supercharged cars, the ECU will lower the amount of pressurized air being fed into the intake.

For the most part, this means that accidentally putting in a lower grade of fuel than is required won’t have any lasting negative effects on your vehicle. You may notice increased fuel consumption and a reduction in performance until the tank is empty, but unless you push the engine hard by either towing or accelerating with your foot to the floor on a regular basis, you’re unlikely to experience any harmful detonation.

If you’re really concerned, you can fill up with the correct grade of fuel once you’ve used half the tank of lower-grade gasoline, which will bump up the overall octane rating of the gas once it’s been mixed together.