THUNDER ROAD: ACROSS AMERICA IN A 2007 FORD MUSTANG SHELBY GT500

Ford has just announced the 2020 Ford Mustang Shelby GT500 will pack an astonishing 760 horsepower and 625 lb-ft of torque, making it the most powerful street-legal Ford ever—with the most power- and torque-dense supercharged production V-8 engine in the world. Back in 2006 we drove the first Shelby GT500 Mustang of the modern era across America, from Los Angeles to New York, where the original Mustang was unveiled 55 years ago this year. That GT500 might have had only 500 hp and 480 lb-ft of torque, but it was still a helluva a road trip.

Crisp blue skies, palm trees and snow-capped mountains sparkling in the distance: LA in its spring glory. And we're rumbling east along I-10 in 500 hp of supercharged Shelby GT500 Mustang, all red-with-white-stripes and Schwarzenegger swagger. Yeah, some days are definitely better than others.

We're on a journey, heading back to where it all began. In six days and almost 3500 miles we'll have crossed a continent, having taken the GT500 from the Pacific Ocean through the heartland of America to the place where, in 1964, the original Mustang stole America's heart: the site of the New York World's Fair. Before we get there, though, we'll have pushed the GT500 to the very edge of its performance envelope on the track and lived through days at the wheel on everything from wide open Interstates to rolling two lane blacktop to the chaos of Manhattan.

Bruce Springsteen's on the stereo, singing about riding out to case the promised land. Thunder Road? Call it the ultimate road test

Our destination this first day is Ford's Arizona Proving Ground near Yucca on I-40. This 3840-acre facility has 50 miles of test roads and is mainly used for truck durability and heat testing. But the road that interests us most is the five-mile high-speed test track - basically, a pair of mile-long straights connected by banked 180-degree turns at either end. It's here that we'll get to really see just how the most powerful Mustang in history really is.

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On the way out of LA, the GT500's manners immediately impress. The iron block supercharged 5.4-liter V-8 under the hood is fundamentally a 4.6 on steroids, but feels an entirely different engine: smoother, more refined and more flexible. The suspension is noticeably firmer than that of a regular Mustang, but it feels toned and athletic rather than harsh and stiff. The fat 255/45R18 Goodyear Eagle F1 Supercar tires up front occasionally snuffle around the grooves in the road, but otherwise the GT500 tracks straight and true with little effort.

It's windy at the proving ground, and tumbleweed debris litters the northern loop of the high-speed track. After a safety check and a brief recce, it's on with the crash helmet and down to business. A tentative launch brings 60 mph in 4.9 seconds and 100 mph in 10.9 seconds, according to our windshield mounted Racelogic DriftBox. The SVT logo on the tach flashes to signal an upshift, but it's way before the big V-8 nears the 6,000 rpm redline, so we ignore it. Little more than 30 seconds after launch the speedo needle kisses 150 mph, then 155, then… nothing. An invisible, electronic hand starts pushing back: like the 2003-2004 Cobras, the GT500 is speed limited. We coax 156.8 mph out of on one run, then 157.5 mph on another. But that's it. Which is a pity, because we're only at about 5,250 rpm in fifth gear at the artificially imposed Vmax, and the engine feels stout enough to go on to the redline - that's 180 mph.

Photographer John Kiewicz, an accomplished drag racer, slides behind the wheel to peel out a few serious standing mile runs. His best nails 60 mph in 4.4 seconds and 100 mph in 9.8 seconds, the quarter mile in 12.7 seconds at 116 mph, and the standing mile in 33.2 seconds at 150.2 mph. Ford's claimed 500 hp at 6,000 rpm and 480 lb/ft of torque at 4,500 rpm - significantly more mumbo than all the pre-launch rumor had suggested - is certainly all present and correct: the 3,990-pound GT500 is just half a second slower to 60 mph than a Corvette Z06, and seven tenths slower over the quarter-mile, despite weighing a hefty 840 pounds more.

We leave I-40 at Flagstaff and turn north-east, overnighting at the small town of Kayenta. Next morning we're rolling through Monument Valley, beloved by Hollywood director John Ford, whose movies such as Stagecoach and She Wore A Yellow Ribbon made the dramatic, weathered landscape an iconic symbol of the old West for millions of Americans. The GT500 feels eager, alive in the cool pre-dawn air, but the dim headlights simply aren't up to the car's performance. After sunrise, though, on the quieter roads in and around the spectacular desert where Arizona, New Mexico, Utah and Colorado meet, the Mustang settles down to an effortless 100 mph cruise, arcing through fast sweepers with a precision that belies its old-school live rear axle.

Out here on the two-lane blacktop, the GT500 feels quick, quiet and composed, everything you want a true Grand Tourer to be. The engine delivers a smooth, linear surge of acceleration every time you squeeze the gas, and while the Tremec six-speed requires a deliberate hand, the shift throws are short. The steering feel is meaty but accurate; the brake pedal is firm but easily modulated. With about 57 percent of its mass over the front wheels, the GT500's natural tendency is to push as the limits of adhesion are reached. As the road tightens, it's best to treat it like an old Ferrari Daytona - brake early and in a straight line, then turn in and accelerate past the apex, using the power to transfer the weight to the rear axle and balance the car.

We head north through the desert to join the I-70 outside Moab, then east, where we eventually climb and twist through the Rockies to 11,158 feet before we begin the plunge down the other side to Denver and the Great Plains beyond. Our destination is the Shelby American Collection in Boulder, a small but stunning group of some of Carroll Shelby's greatest cars. Although the collection is usually only open Saturdays, curator David Murray is waiting patiently for us when we arrive around 8:30 p.m. and shows us around. Treasures include Ken Miles' SC427, probably the most original Cobra race car in the world, the 1964 Le Mans winning Daytona Coupe, John Wyer's personal GT40, and the GT40 Mark IV driven by Mario Andretti at Le Mans in 1967. Naturally there are Shelby Mustangs, too: a three nice GT350s, and 5R002, the first and most significant of all the racing GT350s ever built, found languishing in a barn in Mexico in 1991, and awaiting a full restoration.

Next morning, we roll out of our hotel near Denver Airport and point the Mustang east on I-70 again. For the first time in two days, there are no mountains on the horizon ahead; behind us, the snow-capped Rockies stretch in wall-to-wall widescreen, filling the rear view mirror. Not long after we cross into Kansas we again dive off the interstate, cutting north-east through the plains to Route 36, a near-empty two-lane that basically runs parallel to the Nebraska border and will take us right by the geographical center of the contiguous 48 states. You don't get much more heartland than this: small towns huddle around grain elevators, every second car's a Buick, and the pickup trucks actually work for a living. Well away from the trashy fast food joints and cheap hotels that litter I-70, Route 36 is a glimpse of how America used to be.

The GT500's cabin is proving a comfortable place to be for a 900-mile stint: the high-performance tires roar a little on coarse pavement, and you hear a faint whine from the supercharger overlaying the basso V-8 rumble if you spin the engine past 3500 rpm, but otherwise this Mustang is easy on the ears. The driving position is good; the pedals are well placed, and there's a dead-pedal for your left foot. The steering wheel adjusts for tilt and reach, and though the seats seem a little hard at first, you settle into them. One minor niggle: the seats feature electric fore/aft and height/tilt adjustment, but the backrest is manually adjusted via a coarse ratchet that never seems to click into exactly the right angle.

Mid-morning Day Four finds us at Indianapolis for a rendezvous with an original 1964 ½ Mustang convertible. It looks exactly like one of the three Wimbeldon white pace cars built by Holman & Moody for the tragic 1964 Indy 500 (popular champ car driver Eddie Sachs and rookie Dave MacDonald died in a fiery seven car pileup on the second lap) but it is in fact one of 35 replicas supplied by Ford for use by senior company execs and local dignitaries during race week.  After the muscular heft of the GT500, driving this original four speed manual, 15,913-mile Mustang is like dancing with your grandma: it feels light and spindly and a little frail. The 289 four-barrel under the hood might have been the hottest engine you could get in a Mustang at the time, but it has less than half the power of our GT500. You can almost hear Carroll Shelby sneeringly calling it a secretary's car…