The math seems so straightforward: As engines get smaller and fuel economy improves, the cost of driving will fall, right?
Not necessarily. In a growing number of new vehicles, the methods used to squeeze equivalent horsepower from engines with smaller (or fewer) cylinders have automakers pointing customers toward the pump that dispenses premium-grade gasoline. That's a setback for drivers who hoped their new car's better mileage rating would mean savings with each fill-up.
It's also a reminder that the label of premium, meant to signal the gasoline's octane rating — its ability to forestall the destructive effects of erratic combustion — also applies to the price. According to the Department of Energy, premium averages 30 cents a gallon more than regular-grade gas.
Can't I just use regular unleaded?
It depends whether your manufacturer recommends or requires premium-grade gasoline; check your owner's manual.
- Premium recommended: The engine is calibrated to perform best using premium-grade gasoline, but the electronic engine-management system has a sensor to detect knocking, so it will run on lower-octane gasoline without harm. Drivers might notice a drop in performance and fuel efficiency.
- Premium required: Although the engine-control computer is programmed to protect the engine from damage, extreme driving conditions might cause problems. A manufacturer might not cover engine repairs under the warranty if damage is caused by using gasoline that does not meet specified octane requirements.
It's an extra expense that Micah Highland of La Grange, Ill., knows well. Highland was aware of the Acura MDX's fuel requirements before he purchased the vehicle. Still, when the dealer handed him the keys and reminded him to fill up with premium gas, he thought to himself, "Oh, man. Are you serious?"
The number of vehicles for which automakers recommend premium — guidance typically found in the owner's manual or on the fuel-filler door — is growing. Premium was recommended for only 2.5 percent of all models in 2003, says car-buying-advice site Edmunds.com; for 2013 models, it's 12 percent.
"What we are seeing now is a higher penetration of smaller engines that are developing more power and torque," says Bill Visnic, an editor at Edmunds.com. "As you get to the higher-output engines, manufacturers are probably going to at least recommend premium fuel, if not require it."
Filling the tank with regular-grade gas when premium fuel is recommended has drawbacks. Horsepower, torque and fuel economy might suffer, although modern engine-management electronics prevent internal damage. Sensors in current models detect knock, or detonation in the cylinders, that was heard in older cars as "pinging."
While there's little surprise that luxury cars need high-octane blends, 2013 models whose makers recommend premium include economy-focused vehicles such as the Fiat 500, Mini Cooper and Nissan Juke, as well as some versions of the Dodge Dart, Subaru Impreza and Volkswagen Beetle.
The higher pump price of premium can have a wet-blanket effect on sales of vehicles in the lower price classes. Mazda learned this lesson with its CX-7 compact crossover, introduced in 2007. Originally equipped with a 2.3-liter turbocharged engine that required premium fuel, sales lagged despite the boom in crossovers.
"The whole premium-fuel question is not a problem if you are BMW," says Ruben Archilla, a manager for advanced engineering at Mazda North America. "But it's a big issue if you are trying to sell cars that are in the compact car range."