Pace analysis by AUTOSPORT's technical expert Gary Anderson has revealed just how close the fight is behind F1's leading outfit - which means only a small lift in pace could have big benefits on track.
All teams are expected to introduce significant upgrade packages, but with Red Bull, Ferrari, McLaren, Force India and Williams covered by a smaller gap than the one separating Mercedes from the rest, there is potential for significant movement in the competitive order.
The ranking below, based on the raw pace of each car during the first four race weekends, shows how tight the spread is in the chasing pack, with their percentage pace gaps equated to a lap of the Barcelona circuit, with the Mercedes on a theoretical 1m22.000s laptime.
2 Ferrari, +0.657s (+0.801 per cent)
3 Red Bull, +0.724s (+0.883 per cent)
4 Williams, +0.984s (+1.200 per cent)
5 McLaren, +1.105s (+1.345 per cent)
6 Force India, +1.233s (+1.504 per cent)
7 Toro Rosso,+1.426s (+1.783 per cent)
8 Sauber, +2.179s (+2.657 per cent)
9 Lotus, +2.563s (+3.126 per cent)
10 Marussia, +3.662s (+4.446 per cent)
11 Caterham, +4.198s (+5.119 per cent)
This ranking is calculated by taking the fastest single lap of each team over each of the four race weekends. It is then translated into a percentage of the quickest lap overall and averaged out before being converted into a time deficit.
The above timings are not intended to be a prediction of Spain form, and are distorted by the fact that three out of four qualifying sessions, where teams usually set their fastest laps of a weekend, have been wet.
But what they do show is how tight things are, with Ferrari and Red Bull closely matched on average pace and Williams needing only a net gain of just over three tenths of a second to be in contention to emerge as best of the rest.
Engine performance could be key to dictating any change in the order, with Renault hoping for a step forward this weekend.
"While there are not many visible hardware upgrades, there are significant improvements to the software that should further enhance driveability and therefore overall performance," said Renault head of track operations Remi Taffin.
"Yet again we have also moved forward in energy management and efficiency so we are hopeful our upward trend will continue in Barcelona."
Even Toro Rosso, seventh on average performance could make significant strides if it can find a half-second gain, although this is unlikely in Spain given it is phasing its upgrade package in over the next four races.
Williams is the most improved team this season compared to 2014 and has a major upgrade planned for this weekend aimed at allowing it to continue its strong form.
Along with Mercedes, it is one of only two teams to be faster this year relative to the pace than it was last year, 0.895 per cent quicker than it was in 2013.
This translates to close to nine tenths of a second around a circuit like Barcelona.
"We have got a big step coming in all areas of the car, which is really interesting," said Williams head of vehicle dynamics Rob Smedley.
"But I'm always loathe to say that it is going to put us forward.
"It might just keep us where we are, but if that's what it does, that's good, at least we are developing the car quick enough to keep up with everybody else."
1 Williams -0.895 per cent
2 Mercedes -0.251 per cent
3 McLaren +0.065 per cent
4 Ferrari +0.068 per cent
5 Force India +0.133 per cent
6 Marussia +0.278 per cent
7 Toro Rosso +0.308 per cent
8 Red Bull +0.702 per cent
9 Sauber +1.175 per cent
10 Caterham +1.176 per cent
11 Lotus +2.473 per cent
This uses the figures generated in the same way as those listed above and comparing the average figure for the whole 2013 season for each team with its 2014 average so far.
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