Sometimes small changes in chassis tuning can make the same car either a delight or a disaster to drive. That’s why automotive engineers tick off tens of thousands of miles in cars before things are finalized for sale. It’s in these drives that shock absorber settings, spring stiffness, anti-roll bar stiffness, and tire characteristics are all determined, to say nothing of engine and transmission performance.

Ask any engineer about chassis tuning and he’ll tell you this is the last "black art" in car development. It’s like solving a Rubik's Cube: Modify one component and the characteristics of all the other components change too. Even something that might seem as simple as tires is beyond complicated. There’s such a wide selection available to carmakers, and depending on the car the same tire can be grippy, bouncy, or darty.

So carmakers go to great expense to replicate roads from around the world at their proving grounds, where they do the majority of their tuning. It might sound excessive, but Belgian paving blocks have been imported for GM’s facility in Milford, Mich., Chrysler’s proving grounds in Chelsea, Mich., and the Honda Tochigi test track in Japan. And among the cognescenti, locales like Fiorano, Italy, and Weissach, Germany, have become famous -- because that’s where Ferrari and Porsche test drivers flog cars to extract the best balance and behavior possible.

Yet even with all the emphasis placed on this sort of controlled testing, automotive engineers know there’s no substitute for driving their mules -- that’s what they call the development cars -- out in the real world, on real roads. It’s here too, where a final sign-off locks in the suspension settings before a vehicle goes into production. This sign-off drive usually occurs on the chief or lead engineer's favorite roads, where bounces and jiggles and noise and vibrations are measured through the hands and seat-of-the-pants.

"We don't have instruments that are as sensitive as the human body," explains Mike Neal, long-time Chevy Corvette development test driver.

We asked different car company’s engineers to reveal their secret test roads to us, and compiled the following list.

California

It’s no great wonder that many of the roads mentioned were in California, a state universally regarded as having some of the best driving loops in the world.

Kelvin Hiraishi from Mazda is a sign-off research engineer for the nimble Mazda6 sedan, and he's a fan of California's Highway 74 from San Juan Capistrano to Lake Elsinore, known to some as Ortega Highway, a 50-mile jaunt up and down rolling hills with lots of tight bends. Hiraishi gets more impressions from cars he tests on roads farther north, on the especially scenic California 1, about 100 miles of which connect Morro Bay to Big Sur via twists, turns and spectacular views of the ocean.

Carl Wittman, driving dynamics chief for the Lincoln MKT, says one of his favorite roads for development test driving is California State Highway 74 from Hemet to Palm Desert. He drives this one at night, so as not to be distracted by the scenery. This 60-mile route is two lanes wide, and rises and drops 4,400 feet from its peak near the San Jacinto Mountain State Park to Palm Desert. Another of Wittman's favorites is California State Highway 18 from San Bernadino to Big Bear, which is filled with switchbacks, although it has stretches that are four lanes wide.

Tom Stephens, vice chairman of General Motors, is a hands-on driving afficionado, and says his favorite American road is State Highway 128 in Northern California, beginning north of San Francisco and ending in Mendocino on the Pacific coast. "I really think that road is great for transmission calibrations," he comments.

Arizona

Many car companies have proving grounds located in Arizona, where they can take advantage of both the climate and relative seclusion afforded by the desert. Phil Jansen, Chief Engineer for Jeep, is one of the final sign-off engineers who insures that the driving characteristics of new Jeep models are right. He says his favorite smooth roads for testing are near Chrysler's Arizona proving grounds near Yucca.

Most recently Jansen did work on the new Grand Cherokee, a vehicle that’s supposed to do double duty as a luxury highway driver and as a rock crawler. This presented its own challenges, as the Grand Cherokee needs to be equally well-tuned for both surfaces. Jansen says that smooth roads are best for determining high frequency noises and vibrations, as well as third-order harmonics, which are reflected sound waves. He explains that every part of the vehicle vibrates at its own natural frequency. Each part can be thought of like a tuning fork -- if you hit it, it will make a noise. On the road the trick is to make sure parts don’t amplify other parts. "On rough roads, you don't go fast enough to find these high-frequency noises," Jansen said.

Bruce Robinson is the sign-off engineer for Nissan's thrilling GT-R, as well as the more affordable 370Z. Robinson likes U.S. Highway 191, in eastern Arizona. "It's a beautiful road, in forest lands, with no houses. I think it has 1,000 turns in 100 miles," he says.

This stretch of highway, also known as the Coronado Trail, runs 120 miles between Clifton/Morenci and Springerville. Originally a spur of the U.S. Highway 91, it was known as "The Devil's Highway," when its original number was 666. "I like to run it on weekends," adds Robinson, "But it depends on what car I am driving and what customer the car is intended for."

Other American Locales

Not every favorite test road is located in the Southwest. For Nissan’s more utilitarian cars, such as the Murano, Robinson says he uses the frost-heaved, lumpy roads that wind through the hills north of White Plains, New York. "They really twist the body of the car," as it follows the terrain, he explains.

Robinson also tests off-road abilities, and for Nissan's Xterra he likes the same "Hell's Revenge" section of the famed off-road course in Moab, Utah, that Jeep has long favored.

Jeep’s Jansen says his favorite rough roads are north of Chrysler's Michigan proving grounds. This piece of two-lane pavement gets overstressed by frost and heavy trucks and the general lack of timely road repair in the state. Jansen says there are places on the road that exercise even the lengthy wheel travel of the Grand Cherokee.

Prior to testing cars for Nissan, Robinson used to be a test driver for Buick. He says that his favorite roads back then were the asphalt-on-marshland routes in Louisiana's bayous. Built on soggy land, they exercise a car's suspension like no others, he says.

Outside The U.S.

Unsurprisingly, when we asked for roads overseas, the Nurburgring was mentioned. This amazing 13-mile long section of public road in Germany has 100 corners and plenty of blind hills, earning it a reputation as the world's most demanding race track.

"We like the autobahn in Germany, and the Nurburgring," says Nissan’s Robinson. "We test the GT-R to run at 160 mph and be perfectly calm at that speed; we can't do that on roads here," explains the North American-based test driver.

Stephens’ staff at GM has recently ramped up its chassis testing at the famous course. Since the introduction of Cadillac's XLR sports car in 2004, all new Cadillacs are honed at the famous track. Measuring a car's behavior on this track has even migrated to GM's Buick brand, with tuning on the new Regal also done here. Stephens also mentiones the autobahn, with his favorite stretch being the E54 in Germany headed down through the alps to the E38 in northern Italy.

Johan de Nysschen, president of Audi of America, is heavily involved with the dynamics of Audis built for North America. One of his favorite roads is the 900-mile stretch of his native South African Highway 1 between Johannesburg and Cape Town. "You can really get some speed on that road, and for miles there is nothing else on the road," says de Nysschen.