Motor racing-Hamilton on top, Vettel raises gearbox fear

By Alan Baldwin SILVERSTONE, England, July 9 (Reuters) - Formula One world champion Lewis Hamilton was fastest in final British Grand Prix practice on Saturday, completing a sweep of the three sessions with Mercedes team mate and title rival Nico Rosberg second. While Mercedes were again dominant, Ferrari's Sebastian Vettel had a bad start to a damp day when he told the team over the radio after his final lap: "I think we failed another gearbox". Vettel picked up a five-place grid penalty in Austria last weekend for an unscheduled gearbox change. Hamilton lapped with a best time of one minute 30.904 seconds, 0.063 quicker than Rosberg who missed all of Friday's second practice, on the soft tyres. Australian Daniel Ricciardo was third fastest for Red Bull but more than half a second off Hamilton's pace. Dutch team mate Max Verstappen, winner in Spain and second in Austria, was fourth with Vettel fifth and 1.145 slower than the leading Mercedes. The session was halted to retrieve debris and fix barriers after Marcus Ericsson crashed his Sauber heavily at the exit to Stowe with 20 minutes remaining. The Swede got out of the badly damaged car without assistance, and did not appear hurt in the accident, but Sauber said he was going to a hospital for routine check-ups. Rosberg leads Hamilton by 11 points going into Sunday's race, the 10th of 21 on the calendar. Hamilton is chasing his third successive win at Silverstone and fourth of his career. The two Mercedes drivers are under strict instructions to race cleanly after colliding on the last lap in Austria, their third coming together in five races. (Editing by Amlan Chakraborty)