The Silverstone-based team concedes that this year's car has been weak in that area, which is why it endured such a difficult weekend at Monza.
Fernando Alonso had his least competitive showing of the season at the Italian Grand Prix, struggling to finish ninth, while Lance Stroll was out of the points in 16th.
Alonso has urged the team to address efficiency, and McCullough agrees that it is a priority.
"Fernando touched on the efficiency of the car," said McCullough when asked about the issue by Motorsport.com.
"It is an area where at the start of the year we were definitely one of the slower cars in the straight line.
"We worked on that with the base car, and also the rear wing levels that we brought [to Monza]. It's an area of improvement, and it is an area we need to improve more for next year, that's clear."
McCullough conceded that there are unlikely to be changes that will benefit this year's model, with Las Vegas the key race in the last part of the season that highlights efficiency as much as Spa and Monza.
"The main architecture of the car, the whole car more than just the rear wings, the areas that we've worked on, there are a lot of elements that contribute to the efficiency of the car," he said.
"And from now until the end of the season most of the tracks are not the higher efficiency tracks.
"Our focus is really on AMR24, to make sure that as we're developing that car we are developing as efficiently as we can do, and more efficiently than this year's car."
McCullough added that Monza highlighted how difficult it is to get the right compromise between downforce and straightline speed with the current cars.
"You can choose the rear wing level to be fast in the high-speed grip limited corners," he noted. "It's really important, because the second Lesmo, Ascari and Parabolica have got very long straights after them.
"So you're always trading speed through those corners versus your raceability and straight-line speed.
"Obviously sector one particularly in qualifying is one low-speed corner and just lots of straights, or power-limited sections, that are easily flat.
"I think this generation are cars and the characteristics of people's cars has led to a bit of a difference in rear wing philosophy."
