Drivers have had fill of premium gasoline

The price of premium-grade gasoline has been rising faster than you can say Mercedes-Benz.

Now over $3.55 a gallon nationwide — more than 30 cents above regular unleaded, it’s boosting the cost of owning a long list of cars and trucks, including some non-luxury vehicles.

But as pump prices rise, fewer consumers appear willing to spring for the costlier fuel.

In 2007, U.S. premium gasoline consumption fell to roughly 35.6 million gallons per day, its lowest point in 24 years, according to the U.S. Energy Information Administration.

Experts say the premium pullback is part of a longer-term trend that might be affected by a changing vehicle fleet and the perception by some that the higher-grade fuel is no better for their vehicles than regular gasoline.

Higher pump prices, however, accelerated the trend, they said.

Who needs premium: Premium gasoline is recommended by nearly all luxury automakers and some mainstream automakers as a way to enhance engine performance.

Some automakers, including Mercedes-Benz, require premium gas because they say using lesser-grade fuel can damage engines and invalidate warranties.

Whatever benefits premium might provide in high-performance cars, it doesn’t necessarily follow that cars designed for regular will benefit from premium.

“Using a higher-octane gasoline than recommended by the owner’s manual does not improve performance or fuel efficiency; it only costs more money,” the U.S. Environmental Protection Agency said in its 2008 Fuel Economy Guide.

What’s the difference: Premium gasoline carries an octane rating at or above 91, compared with 89 for midgrade and 87 for regular unleaded.

Fuels with a higher octane rating are more resistant to premature detonations, which a driver hears as pings and knocks. These waste energy in the fuel and can harm the engine.

The risk of premature detonation is higher in vehicles with high-performance or high-compression engines — sports cars, luxury sedans and SUVs.