McLaren appear to have made up some ground on their rivals in certain areas, but Jenson Button says their straight-line speed in Mexico shows they still have a tough job ahead.
Button and team-mate Fernando Alonso have been well of the pace of the front runners this season as Honda has struggled with both pace and reliability, but they have taken a step forward and were only 0.3s slower than Mercedes in the last two corners at the Autodromo Hermanos Rodriquez last time out.
However, that pace was negated once they hit the straights in Mexico and Button, who found himself running in the top 10 early on before dropping to P14, says they need more speed on the straights.
"The car was working better there [in the slower sections]. When [Williams' Valtteri] Bottas lapped me, I dropped back one-and-a-half seconds, and I actually caught him and got into the DRS zone," he told Sky Sports News HQ.
"Those sort of sections it's working pretty well. High-speed was okay, still needs a bit of work, but the very high-speed bit without turning the steering wheel [down the straights] is pretty tricky.
"You can't do anything. I'm making myself as small as possible in the car, but we've got a lot of work to do."
McLaren have suffered 15 retirements this season and Alonso's Mexican GP lasted only one lap after the team picked up a problem with the Honda engine after qualifying, but Button is hoping they will take a few steps forward in the final two races of 2015.
"We've lost more power than most at altitude for some reason, I don't know why. Normally you'd think it'd bring everyone closer together. But it really hurt us here.
"Hopefully Brazil will be a bit better, then Abu Dhabi, and then we can enjoy the winter and look forward to next year."