Not just an empty tank and cold weather.
http://www.bridgestone-eu.com/articles.asp?ID=50591
Hisao Suganuma, Bridgestone Motorsport's Technical Manager was particularly pleased
commenting: “Whilst much of the credit for yesterday's record breaking time by Michael should rightly go to Ferrari, it was also an exciting culmination of the huge amounts of hard work put in by all the Bridgestone teams and our tyre engineers. What's more, Michael's time came at the end of a simulated race run which shows the competitiveness of the Bridgestone tyres from start to finish.
Obviously we don't know just how competitive we will be until the first race in Melbourne next Sunday but recent results suggest our rivals will have a battle on their hands. What's more, there will be further developments to come throughout the season.”