Maverick Vinales and team-mate Valentino Rossi were united in urging Yamaha to squeeze more top speed from next year's M1 engine at the close of 2019 MotoGP testing.

Vinales has put the latest Yamaha prototype – including a new frame, airbox and engine - on top of the timesheets during both post-season tests at Valencia and now Jerez.

However, the bike's biggest weakness remains top speed and neither factory rider feels confident that the third evolution of the 2020 powerplant made big enough gains against the likes of Honda and Ducati.

"The feeling is good. The lap time is there. But again… if we don’t start first, we will get in trouble!" Vinales said, referring to the difficulty of trying to fight faster bikes on the straights.

"Yamaha is working really hard, but today we see again we are 10ks [down] on top speed compared to Ducati. So we have to work harder.

"For sure in many other areas we are quite strong, but still in the weak point, we are at the same level right now.

"Maybe we improved a little bit at other tracks but here in Jerez it's the same [top speed difference]. So we need to keep concentrating and see what we can do for the next test in Sepang."

While Vinales set the best lap of the test on day one, Rossi improved before the rain arrived on day two, albeit finishing eleventh overall and 1.221s from the Spaniard.

Afterwards, the Italian echoed his team-mate's top speed concerns.

"For me the new engine is on the good way, but still the gap is very big because if you lose 9-10km/h at Jerez, when you have a track with a really long straight it's difficult," he warned.

"We feel something [from the new engine] but we need more. I hope that Yamaha are able to improve a little bit for next year."

Nonetheless, Rossi was encouraged by Vinales' form plus his own performance on the final morning.

"For me, Maverick is always fantastic in the test… able to [push] even more than all the other riders. This means that anyway the bike is good," Rossi said.

"So from one point of view we are happy, but we need anyway to stay concentrated because we have a lot of work to do before the next season.

"And for me personally, yesterday we worked more on the settings, we tried some different things, but I was not happy. But today, in ten laps, I was already good. I am half a second from the best lap time but a lot closer than yesterday.

"It was a shame about the weather because we had work to do to make a comparison between the new bike and the old bike… But after the rain we did some work with the new bike in the wet. It was not so bad."

The final version of the 2020 Yamaha engine is expected to be delivered for the next test at Sepang in February.

However, Yamaha team director Massimo Meregalli warned there is unlikely to be a radical step.

"Riders are always looking for more. But what we were looking for was to increase the smoothness, and we did. And to increase the top speed, and we did," Meregalli said.

"I think the area where we should try to do another step is the top speed and we are expecting we might receive an update in Sepang.

"But the base of the engine for next year is more-or-less this one."

Satellite Petronas Yamaha riders Fabio Quartararo and Franco Morbidelli, who received the latest engine but not the new chassis or airbox, were fourth and seventh quickest at Jerez.