Greg Biffle topped the times in both of Saturday's two practice sessions for the Daytona 500, making him a firm favourite for pole position going into Sunday afternoon's qualifying.

Biffle set a time of 46.537s (193.395mph) in the first two-hour session, faster than Marcos Ambrose by just 0.011s on the 2.5-mile tri-oval. In the afternoon session that ran for one hour and 45 minutes, Biffle's closest competition was last year's race winner Trevor Bayne.

Biffle gave the credit to his pit crew and to crew chief Matt Puccia: "It's a testament to how hard these guys have worked because here the driver really doesn't do anything in qualifying," he said. "So it's Matt and all these guys on the team who work at the track and the shop. They've worked really hard on these cars and the motors have been awesome."

But while Biffle agreed that it made him one of the favourites going into qualifying on Sunday afternoon, he also had some concerns.

"We're definitely one of them, but some other guys still have some speed left. The thing that concerns me a little bit is that's about all we've got," he said. "If guys come out and pick up a couple tenths, they're gonna put some pressure on us because look at how tight it is. It's so close right now, but we're one of them. We're gonna be one of probably eight cars that has a stab at it."

Qualifying is due to start at 1.10pm ET (6.10pm GMT) with the order of the timed runs being set by lottery. A field of 49 drivers will be attempting to make the grid. There is some concern of rain affecting proceedings, in which case qualifying could conceivably be pushed over to Monday.

Running a second slower than Biffle, Jimmie Johnson was at least running in practice once his Cup car finally cleared inspection on Saturday morning, after new pieces for the #48 were fabricated at the Hendrick shop in North Caroline and flown down to Daytona Beach overnight. NASCAR officials confiscated 'C-posts' from the car on Friday after they were found to be out of tolerance in the initial inspection, and it looks as though Johnson's crew chief Chad Knaus could face serious penalties for the transgression.

Before Daytona 500 qualifying, Biffle and Johnson will be among the 25 competitors in Saturday night's Budweiser Shootout. Ambrose will also be in the field, but Trevor Bayne will not even though he's eligible through his Daytona 500 win last year; his Cup team, Wood Brothers, does not have the funding to enter him into the event.

Full practice times are available.