What’s with all these long runs in the F1 tests? It’s all down to the tyres.

The first two Formula 1 pre-season tests seem have included more long runs than in previous years, but why? I’d say that it’s down to the refueling rule changes for the 2010 season as well as some of the technical regulations around the size of the tyres.
Last year with the re-introduction of slick tyres the teams kept the same size tyres that they had in the grooved days this meant that proportionally the front tyres gained more grip. This was because both front and rear had four grooves that were now part of the contact patch, with the rear tyre being wider it meant that the four grooves worth of grip supplied was less in percentage terms than returning the four grooves to the front. This change upset the balance of the car with the front now being more grippy than the rear. The 2010 regulations revert this balance by [...]