Topping the lot was the newly Mercedes-powered Williams' Felipe Massa, who tripped the radar at 286kph on the front straight, and 307 on the fastest part of the southern Spanish circuit.
In contrast, the nearest Ferrari-powered car was the works F14-T driven by Kimi Raikkonen, with just 268 on the straight and a top speed of 289 -- considerably down on the leading Mercedes-Benz machine.
Auto Motor und Sport's Michael Schmidt said Ferrari has admitted it did not deploy full power at Jerez, where the all-new and sophisticated V6 'power units' were getting the first real running.
Mercedes' Niki Lauda, however, said there was no sandbagging in the silver camp.
"We went to the limit that we could, as it makes no sense to go deliberately slowly. You learn nothing (from that)," said the great Austrian and Mercedes team chairman.