Most of us are still saying premium or unleaded to distinguish between these two types of gasoline. But take note that all gasoline types are now unleaded so it's misleading to call it that way.
So what's the difference? Premium has higher octane, and Regular gas has lower octane. Again, both are unleaded.
But here's another wrong notion. Many believe that fuel with higher octane gives more power than those with low octane rating.
FACT: Octane rating is not for POWER. It is formulated to match the compression ratio of certain engine type. Regular vehicles has low compression ratio and therefore doesn't need high octane fuel. These engines are designed to use Regular gasoline.
While sports cars have high performance engines and the compression ratio of this engines are very high. Using regular gas for these engines will cause pre-ignition and will surely damage it.
So what does octane really does in the fuel? It's just a retarding agent to prevent pre-ignition caused by high compression in sports engines. That is why high performance engines should only use premium gas (which we wrongly call "leaded").
-Low octane gas is for low compression engines.
-High octane gas is for High compression engines.
If your vehicle is not a sports car, don't use premium gas because it has no effect on the power whatsoever and you only end up wasting money for something you don't need.
And if your vehicle is a sportscar with a high performance engine, don't use regular or you will damage it.
On more thing: Because all gasoline fuel are now unleaded, don't say Premium or Unleaded. Say premium or regular.