Honestly, as much fun as it would be to see Clarkson in a Sprint Cup car (melted Goodyears, yes, but only after he was shocked by the sheer brutality of the acceleration), there's three good reasons it's Hammond in there.
1) I'm not sure Clarkson could get *into* a Cup car. He's as tall as the tallest NASCAR driver I know of (Michael Waltrip), but I'd guess he's at least 50 pounds heavier; I'm not sure he'd be able to fit through the window opening, and remember, those cars don't have doors.
B) While James would be entertaining in a NASCAR machine (remember how much he *loved* the Jag racer?), honestly, he's probably the one who'd be the best behind the wheel there--remember, Jackie Stewart thinks that Captain Slow is actually the best driver of the three--and really, there's no way that they could pull the "getting lost" gag on an *oval* track like Texas Motor Speedway!
III) Always remember, Hamster is a closet American.
Also, I suspect that TG is doing some NASCAR stuff while in the States because NASCAR is starting to get more exposure worldwide, thanks to cable and satellite networks like Speed that cover nothing but motorsports. It's the most popular form of racing in the States, and its relative simplicity probably appeals some to Jezza's "don't give me computers between me and the road, just give me MORE POWAAAAAH!" side. Seeing as how when they shot that segment (last fall), NASCAR still prohibited on-board computers of any sort, he'd like that! (For the record, starting with the 2012 season, NASCAR has joined the late 20th century in adopting computer-controlled electronic fuel injection instead of using a Holley 830cfm four-barrel carburetor. The ECU is a unit designed and built by McLaren, by the way!)
That, and there were logical tie-ins. For their latest US road trip, they started in the heart of NASCAR country, the area where the bootleggers and moonshiners were building extremely quick cars to outrun the police on deliveries, and started running races in empty fields just to see who really did have the best car. Since it's such a key part of the area's history *and* the area's economy (Concord, NC, and environs, just outside of Charlotte, basically relies entirely on the fact that almost all top-level NASCAR teams are based there), you can't really leave it out--not to mention that North Wilkesboro, a track beloved by NASCAR fans that has sat pretty much idle since 1997 (long story), offered an irresistible opportunity for first-class hoonery without annoying anybody--no closing an airport for a day, no closing public roads, just a nice empty racetrack they could be utter children at!
As for the upcoming one, it was shot this past October, when NASCAR's top two series, Sprint Cup and the Nationwide Series (and no, that's not them being braggarts, it's named for title sponsor Nationwide Auto Insurance; used to be the Busch Grand National series), were at Texas Motor Speedway for their fall races there. What's the connection? Well, History Channel bought the title sponsorship of the Nationwide Series race to promote TGUS, and thus it was the "Top Gear 300." While it was the TGUS boys who were there to be grand marshals and such, I can't blame Andy for wanting to get a TGUK segment out of it, too--NASCAR has always prided itself on being particularly open to the media and willing to help out anything they see as potential good publicity, so you knew that they would throw the doors wide open for the world's most popular car show. (Hell, technically, they had to waive one of the Sprint Cup series rules for the segment--NASCAR prohibits teams from testing at any track used by a NASCAR touring series, except for teams invited to participate in Goodyear tire testing, which may only participate in the Goodyear tests and can't bring any of their equipment to work on setups, and an annual preseason test at Daytona in January. Hamster driving the race car at Texas would technically be considered a test session under NASCAR rules, so they had to waive that rule for the shoot!)
Oh, Nascar finally figured out what fuel injection does eh? Nice. What surprises me is that they went to McLaren instead of Holley, as Holley already does EFI quite well...and make kits specifically for converting old, 4bbl muscle car engines to it.