Key Biscayne drivers are paying $6.99 a gallon for premium gasoline at the island's only two fuel stations, 1 cent short of $7, as of Monday, Sept. 28.

Both the Mobil and Shell stations held regular at $5.99 a gallon, the highest price in the Miami area, the Key Biscayne Portal reported. The cheapest Miami station was selling regular for $3.89 on Sunday, Sept. 27, according to fuel-price tracker GasBuddy. That gap: $2.10 a gallon.

Island prices have climbed since midsummer

Key Biscayne prices have climbed steadily since midsummer. On July 20, regular at both stations stood at $5.49. A week later it had jumped to $5.79. At that point, the cheapest Miami station was $3.43, putting the island spread at $2.36 a gallon.

Regular stayed at $5.79 through mid-September. Then, as Islander News reported Sept. 21, the Mobil station raised its price to $5.99, and Shell followed the next day with the same 20-cent increase. The two stations have matched each other's prices throughout the summer and fall. Key Biscayne was already the most expensive place to fill up in the Miami metro area that week.

The $6.99 premium figure is a new data point. Earlier reports tracked only regular-grade pricing.

Miami averages rise as national prices dip

Across Miami, the average gallon of regular hit $4.38 to start the final week of September, up 41.1 cents from a month earlier. That price was also $1.43 higher than a year ago, according to GasBuddy's survey of 1,690 area stations. Miami's average rose 6.5 cents over the prior week alone.

The national picture moved in the opposite direction. The U.S. average fell 1.6 cents to $4.42 a gallon as of Sept. 28.

Analyst sees possible relief, with caveats

Patrick De Haan, head of petroleum analysis at GasBuddy, said oil prices below $100 a barrel could bring "modest declines ahead for gasoline," but warned "the broader geopolitical situation remains fluid and could push prices in either direction with little warning."