Both Webber (right) and Vettel have completed race-distance simulations
By Ted Kravitz
BBC F1 pit-lane reporter at the Circuit de Catalunya
While absolute pace is hard to read in off-season testing, there is a parameter that is simple to judge - whether a team is having a good test or a long, hard slog.
World champions Red Bull are clearly having a very good test here in Barcelona.
The car is reliable, which allows the team to focus on understanding the Pirelli tyres and learning how to get the best out of them, while managing their particular foibles.
Mark Webber has taken over from Sebastian Vettel for the final two days of the test and both will have completed race simulations by the time they leave.
That will give the team confidence going into the first race, wherever and whenever that will be.
One area of the RB7's design that catches the eye is the rear bodywork.
The car tapers away from the sidepods back to the rear wing in an uninterrupted slope. The cooling is achieved by venting the hot air through one fairly large single outlet which blows very neatly on to the rear wing.
McLaren's Kers reliability has been exemplary - which is more than can be said for the rest of the car
exhausts are cloaked in a sleeve that exits
above the diffuser, further adding to the clean lines.
The Red Bull front wing is similar to last year's version, but expect to see a development for the first race.
One thing we have not heard much about is how the Red Bull's Kers energy recovery and power-boost system is working.
Unlike McLaren, Ferrari, Renault and Sauber, Red Bull have never run Kers before, so they are far less experienced in this area than, for example, McLaren.
The system McLaren used in 2009 was widely regarded as the best in F1.
That is why Jenson Button has said that if McLaren are to have an advantage anywhere, it will be in their Kers management and reliability, which has been exemplary. Which is more than can be said of the rest of the car.
Mercedes had a Kers failure on Nico Rosberg's car on Friday, and the German spoke afterwards of the vicious circle that can develop with Kers and tyre wear.
Put simply, as the rear tyres wear out, it gets harder and harder to recover the energy from the car during braking without upsetting the car's balance.
The Williams's tiny rear end has attracted a lot of attention from rivals
The rear wheels lock up, which damages the tyres more and forces the driver to start fiddling with the brake balance on top of everything else he has to do in the cockpit.
On the subject of tyres, the extent to which the tyres wear out is evident in the amount of rubber that's flown off them round here.
Cars with white sidepods, such as the Force India, often come back into the pits after a long run with chunks of rubber splattered onto the rear bodywork.
Tyre degradation also means the return of a well-known F1 phenomenon - marbles.
These are little balls of rubber that settle off the racing line, ready to upset the handling and reduce the grip of any car that runs over them.
The Barcelona track is collecting marbles to the point that it has to be swept twice a day, including in the daily lunch break.
In recent years the Bridgestone tyres, being harder than the Pirellis, tended not to produce marbles in the race, so avoiding them this season will be another skill for the drivers to master.
Other teams that have caught the eye so far include Toro Rosso, Williams and Sauber.
The Toro Rosso is quite a step forward from the team's 2010 car
Sauber have a Ferrari engine and gearbox and therefore a similar rear suspension layout, too.
The car is proving just as effective in looking after the tyres as the Ferrari and Kamui Kobayashi told us that he expected to be able to fight for points-paying positions this season.
Toro Rosso have built a fabulous-looking car, with beautifully rounded sidepods that sculpt themselves round the radiators. It is more revolution than evolution for the team, as they move further away from imitating the designs of their sister team Red Bull Racing.
Jaime Alguersuari finished day two just behind the Ferrari and Red Bull and he insists that performance was not set on a light, qualifying-level fuel load. The Spaniard added that he hopes and expects the Toro Rosso to be quicker than Sauber and Force India this season.
Williams are expected to be in that midfield pack, and they, too, have adopted an aggressive approach with the rear of their car, in particular the gearbox, which is one of the smallest seen in Formula 1 for some time.
Like Red Bull, the idea is to improve the airflow to the rear wing.
The rear wing itself is supported by a single pillar that is mounted centrally, on top of the gearbox, and this has attracted a lot of attention at Barcelona.
It seems to have quite a high degree of lateral movement. When the car runs over a bump or kicks its tail out in the pit lane, the whole rear wing assembly appears to shudder from side to side to a greater degree than other cars.
It will be interesting to see how it performs when it matters.