Whisper it gently, but this might just be another good day for McLaren. They led the way yesterday, and being McLaren, also broke down twice.

But today Carlos Sainz has just bounced to the top with a 1:18.878, narrowly ahead of Sebastian Vettel's Ferrari with a 1:18.909 and Max Verstappen's Red Bull with a 1:18.979.

So just over one hundredth of a second separating our top three.

You'll read about me waffling on about C1, C2, C3, C4 and C5 today. They are the tyres that we have at our disposal for testing, as well as the inters and wets (and randomly a developmental tyre).

Gone is the rainbow system of tyres. That's so 2018. In its place, well, for testing anyway, is the C1 (white, no brackets) which is the hardest, the C2 (white with brackets) the second hardest, the C3 (yellow, medium), C4 (red with brackets, second softest) and C5 (red, no brackets, softest).

Er, sorry for confusing you further. C5 is most definitely the softest option out there.

Image source, Pirelli

Stuart Oakley: I call extreme sandbagging from Mercedes; at least that's what I'm hoping.

Eleventh and 12th out of 13 yesterday and fifth of six so far today for the world champions. Still wouldn't be too concerned just yet though.

Ah, I see what they did there.

Andrew Benson
BBC Sport’s chief F1 writer

After the well publicised delays of the first test, Williams finally got some decent mileage in at the start of the second, George Russell racking up an impressive 119 laps on Tuesday. The British rookie said: “After the last couple of weeks it's a huge relief to get a day like that. We've learned so much about the car at lower fuel and higher fuel. It gives us such a good indication moving forward into the next few days, and for the guys back at the factory who are already designing the upgrades.

"I think everybody's happy with the performance - not in terms of lap times but in terms of no issues. Everybody's been working so hard. We're all human. Everybody is pretty knackered. They've been performing 24/7 over the last two weeks to get us ready.

"It certainly hasn't made up for last week because everyone else at this stage is fine-tuning the balance, whereas we’re needing to understand the car. But I feel we're in a good spot. The car isn't in a perfect window right now but I feel like it's definitely got the potential."

Image source, Getty Images

All teams have had some time on the track today, which is always a good start.

Five of those drivers have set times. Max Verstappen remains the quickest, but has gone even faster with a 1:18.979 on the C3 medium compound tyres.

Sebastian Vettel and Kimi Raikkonen are both on the same tyres and are second and third respectively. Valtteri Bottas and Robert Kubica have started on the C4s (the second softest compound and are fourth and fifth).

Kubica, though, has only recorded a best time of 1:48.157, nearly 30 seconds slower than Verstappen. More problems for Williams or just trying out some things before it gets warmer?

As always you have your chance to tell the world your feelings. Tweet us using the hastag #bbcf1 to get involved.

Chris Medland
F1 journalist in Barcelona

There was plenty of interest around Mercedes on Tuesday morning as the defending champions introduced a heavily-updated car, with Lewis Hamilton enjoying a productive morning with the upgrades.

The same couldn't be said of Valtteri Bottas, who completed just six laps in total as a result of an oil leak that lead to a power unit change.

The Finn has already carried out more work this morning though with nine laps, and it's more aerodynamic testing so far. Right now it looks like Mercedes are focusing on how their updated bargeboards affect airflow to the diffuser, with plenty of sensors in front of the rear wing.

Image source, Chris Medland

Andrew Benson
BBC Sport’s chief F1 writer

It has been, it has to be said, an unusual pre-season testing period so far for Mercedes. In week one, the world champions were off the pace, it was as simple as that, and they weren’t bothering to hide it. For week two, they arrived on Tuesday with a massive upgrade package that amounted to something like a B-spec car.

They spent the morning assessing what is intended to be pretty much their Melbourne car - although new parts are coming through the rest of the test - but attempts to collect sufficient data foundered with an engine problem in the afternoon. That meant fewer than 100 laps in a day - very much not standard procedure for a team that usually runs like clockwork.

Technical director James Allison admitted: “It (Tuesday) was not exactly the day we hoped for, running for the first time with the bodywork kit that we expect to use in Melbourne, with a large part of the day lost to an oil pressure issue at lunchtime. Nevertheless, either side of that problem we did do some useful things, finding that the car behaves a little differently with the new package. We're looking forward very much to have a trouble-free day to get a better feel for what it can do.”

Lewis Hamilton was the only Mercedes driver to do any significant mileage. He said: “The car was feeling relatively good. It was an improvement from last week which is a good step.”

Image source, Getty Images

Bottas, by the way, has already done nine laps. More than he managed all of yesterday.

And The Lion is roaring early on, with Max Verstappen (on the C3 medium compound tyres) sending in a fine-looking 1:19.983 early on, a time better than anything Daniel Ricciardo. Valtteri Bottas, Lewis Hamilton or Nico Hulkenberg managed all day yesterday.

Bottas, looking for some much-needed time in the car after only managing seven laps yesterday, has also set a time, but is more than two seconds slower than the Dutchman with a 1:22.624 on the softer C4 tyres.

"Daniel is back in the seat this morning. We have two aero rakes on the car for the early running before Daniel will focus on a race simulation."

We're already under way in Barcelona and a number of drivers are already out on track, including Valtteri Bottas (Mercedes), Romain Grosjean (Haas), Kimi Raikkonen (Alfa Romeo), Robert Kubica (Williams) and Sebastian Vettel (Ferrari).

We're also expecting to see Max Verstappen (Red Bull), Carlos Sainz (McLaren), Sergio Perez (Racing Point), Daniel Ricciardo (Renault) and Daniil Kvyat (Toro Rosso)

But Hamilton's team-mate Valtteri Bottas had limited running because of an oil leak.

World champion Lewis Hamilton seemed happy enough.

But some people had problems. Alexander Albon had "a couple of issues" but still had a productive day.

Image source, Toro Rosso

Don't forget George Russell. He put in a shift too.

Antonio Giovinazzi was also working from sunrise to sunset.

Pierre Gasly was second quickest after a huge stint.