All the evidence suggests that, during testing at Jerez last week, the big teams were brimming their fuel tanks before their long runs and keeping a decent amount on board even for short stints. The fast cars simply weren't fast enough for that not to be the case.
Last year's best time around there was a 1m17.4s, set by both Red Bull and Williams. But last week no one bettered 1m19.5s. Okay, the 2010 cars, with their bulkier tanks, probably won't be as aerodynamically efficient as their '09 predecessors, and those narrower front tyres won't have as much grip, but you can be fairly certain they're not going to be 2s per lap slower.
Fernando Alonso and Lewis Hamilton each did runs that would have required around 120kg of fuel. If you made allowance for that, they were in the high 19s-early 20s. But let's assume Ferrari and McLaren had each brimmed their tanks before those runs, squeezed in around 170kg (the widely-expected total capacity of the 2010 cars). In other words, they would still have had 50kg in their tanks when they stopped - around 1.58s worth. Then the times would make much more sense.
Having the fastest 2010 cars on low fuel capable of lapping in the high 1m17s/low 18s would tally much better, not only with the '09 times but also with the Toro Rosso's speed last week. Sebastien Buemi did the weight-adjusted equivalent of a 1m19.4s (actually a 1m20.0s, but at the beginning of an eight-lap run) - which would have put the little team's car just a couple of tenths adrift of McLaren and faster than everyone else. Doesn't quite tally, does it? But it does if you have him on genuinely low fuel and Ferrari/McLaren always with at least a spare 50kg.