The price of premium-grade gasoline has been rising fast and is now over $3.60 a gallon nationwide — more than 30 cents above regular unleaded — the high-end fuel is closing in on the $4 mark, boosting the cost of owning a long list of cars and trucks, including some non-luxury vehicles, the Houston Chronicle reports.

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.

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, even require premium gas because they say using lesser-grade fuel can damage the engine and may invalidate warranties.

But whatever benefits premium may 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.

Premium gasoline carries an octane rating at or above 91, compared with 89 for mid-grade 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, like those in sports cars, luxury sedans and SUVs.

Mercedes owners who ignore the company's 91 octane fuel requirement could get increased deposits in their vehicles' exhaust systems, said Rob Moran, a spokesman for Mercedes-BenzUSA in Montvale, N.J.

Using lower-grade fuel also could potentially void the car's warranty, he said.

Other automakers are more flexible. Volvo, for instance, recommends using 91 octane gas, but says customers should notice "little or no difference" if they fill up with a lower-grade fuel, said Steve Hansen, manager of product strategy for the Swedish automaker's Irvine, Calif., office.

Modern auto engines, using electronic sensors, adapt to different fuel grades, he said.

Sean Lobosco, a BMW spokesman, agreed that the sensors would help protect a high-performance engine using a lesser-grade fuel.

But the cheaper fuel could "gum up" the motor and would reduce the horsepower and torque levels the vehicles achieve when premium gas is in the tank, he said.

According to the Houston Chronicle, oil refiners say the national figures are consistent with what they are seeing at their gasoline production facilities.