MIAMI BEACH—Ernesto Evangelista prefers to pump premium gas into his seven-month-old Nissan Titan, thinking it makes the truck run better.

But at a BP station, the 33-year-old painter grabbed the handle for the regular, 87-octane gas to fill his tank on a recent Friday.

“Premium is just too ex-pensive,” he said. “Nobody can afford to fill up with premium anymore.”

With rising fuel prices pushing the national average for premium to $4.48 a gallon—about 40 cents higher than regular—motorists like Evangelista are buying less of it, industry statistics show.

Demand for high-octane fuel is at its lowest in nearly a quarter of a century and is now primarily consumed by a core group of luxury vehicle owners—and even some of them are putting lower-grade fuel into their tanks to save money.