Daniel Ricciardo says Red Bull is unlikely to use Renault's new-spec Formula 1 engine for this weekend's United States Grand Prix as it is "probably not worth taking".
The French manufacturer has spent a "significant" number of the 12 development tokens it had available on an upgrade, making it available to customers Red Bull and Toro Rosso from this weekend.
With Autosport led to believe the performance gains are only worth around 0.15s per lap, the suggestion is Red Bull will opt against running it.
Ricciardo said: "It's available if we want to use it, that would mean a penalty though.
"We've got to understand if it is worth it. Last I heard it is not massive.
"My understanding is it is probably not worth taking.
"If we start from the back I don't think we'll make up the ground we would need.
"Right now we are less likely to take it. That's about it.
"Hopefully today they'll tell me the difference and we'll find a bigger chunk of horsepower from it, but realistically it's not going to be worth a lot."
Carlos Sainz Jr has confirmed Toro Rosso will not be running the system at all for the rest of the season, with Mexico, Brazil and Abu Dhabi following Sunday's event in America.
Asked by Autosport whether the team would take the new Renault engine, Sainz replied: "No. We're going to stick for the rest of the year with the old spec.
"I don't think, given the values that they've given, it's worth getting another grid penalty and to start last.
"These last four tracks are similar to this one [Austin]. Not the best tracks, as they have long straights, but not the worst.
"So if we can be there, close to the top 10, top eight on some of them, to start last is too much. It isn't worth it.
"We will commit to this year's spec that we have now. It's good enough to finish the year and we have plenty of them."
It is understood Toro Rosso is to switch to a one-year-old spec Ferrari unit for next year, notably with the four engine manufacturers and the FIA recently agreeing to a rule change that would allow such a circumstance for 2016.