Red Bull's choice of fuel supplier means it is not maxing out the power of the Renault engine, according to Cyril Abiteboul.
The long and often troubled Red Bull-Renault partnership is entering perhaps its final days; Red Bull is tipped to switch to Honda power for 2019, but is currently refusing to honour a decision deadline imposed by Renault.
Amid that delay, Renault boss Abiteboul said that Red Bull's choice of fuel supplier has not helped the situation.
"You can get extra power from the fuel but we can only guarantee performance equality with what we have in our hands," he told Auto Bild.
Renault and its customer McLaren use BP-Castrol fuel, while Red Bull works with Exxon-Mobil.
"For strategy and marketing reasons, they have their own partner, which we support," Abiteboul insisted.
"I'm sure Exxon has the ability to catch up. But right now they are a bit behind."
The calendar continues on Sunday with the French Grand Prix.