Formula 1 started 2013 with the first test in Jerez. With limited testing, every testing session is vitally important, and teams have to make use of every test opportunity they can get. Although it is impossible to judge how the teams have improved over the winter in a test, everyone still tries to make some sense of the lap times. Ferrari set the fastest lap time on soft tyres in Jerez, and Red Bull set the fastest time on the hards. This is the only important fact of the whole test session. Ferrari has recovered, and their car is  not a catastrophic disaster as it was last year, and Red Bull is still very fast.

Romain Grosjean in the Lotus

Jerez has an extremely abrasive track surface, so the long runs on a heavy fuel load also do not have relevance whatsoever. The soft tyres barely last for 5-6 laps. So tyre data collection was corrupted somewhat. The other headline facts of the test are,

• Marussia, under technical director Pat Symonds now has KERS and the Coanda exhaust system.
• Caterham is trying to wing this year, and is going to divert all its resources to 2014. They probably want to simply finish in the points this year and keep ahead of Marussia.
• Torro Rosso looks quick, and this improvement is due to their new technical director James Key. Also, the Torro Rosso is called STR8, funnily enough. Torro Rosso might switch to Renault engines in 2014 so that they can use Red Bull mechanicals in the future.
• Sauber has made their smaller sidepods work, and hasn't had any cooling issues. Smaller sidepods mean less drag, so Sauber has had to package their internals very well. Yet. The real test is Barcelona, where the cars will really be driven in anger. But Sauber looks fast as well, and with Nico Hulkenberg at the wheel, they will challenge for podiums and wins.
• Williams ran their 2012 car, so no one knows how fast their 2013 car is. But Valtteri Bottas, another Kimi Raikkonen, had his first taste of how the team works in race simulations.
• Force India looks strong as well, and they have yet to confirm their second driver. It's definitely Jules Bianchi.
• Mercedes had bad time at Jerez. Rosberg had an electrical problem on the first day, and within 15 laps, the Merc was on fire. They had to fix a fundamental electrical design problem, and missed the entire first day of running. The second day, Hamilton binned the car at the fastest part of the track as his rear brakes failed on his 16th lap. Hamilton managed to slow down the car slightly by locking up his fronts, but still smashed the front wing and nose after crossing the gravel trap. And so another day wasted. Meanwhile in the pits, everyone at McLaren were rolling on the floor and laughing. But no one noticed them, as everyone else in the pit lane was also laughing quite a lot. Mercedes recovered on the third and fourth day of the test, clocking on an average of 145 laps each day. Thats a lot compared to other teams who barely did a 100 or so laps. Mercedes also realized that when Hamilton drove hard, the car took quite a beating. When he drove into the pits after along stint, the brakes were smoking, the engine was complaining, and so on. 
• Ferrari has a good car, and Massa and Pedro de la Rosa drove it in Jerez. Interestingly Alonso did not opt for the Jerez test. As Massa completed lap after lap of aero test, engine test, and other such tests, he could not work on setup. When Alonso drives the car in Barcelona, the cars will be ready to use and Alonso can start work on setup right away, and won't have to do the boring work of checking all the systems and collecting data. Jerez is not a track that has similar characteristics to any of those on the calendar, so it's not ideal to setup a car, or even train on. So Alonso has cleverly opted to only run at Barcelona
• Lotus, under technical director James Allison also has a very fast car. Romain and Kimi, both seemed happy with it. Although they didn't run their passive DRS in Jerez, the car was relatively reliable as well. All teams will bring their final updates and new parts in Barcelona, so Barcelona will be a real test to gauge the speed of the cars.
• McLaren has a very fast car, and Jenson set the most important lap time of the test. He set a very fast lap time on a relatively green track, on the first day on hard tyres. So the McLaren is quick as it was last year.
• Red Bull was running different array of tests throughout the session, and so neither Vettel nor Webber pushed the car hard. But they are definitely quick, and more importantly, very consistent. Their longer runs were the most consistent of any team. So they remain favourites.