How Fast Can a Car Go?

In the vacuum of space, only the speed of light limits how fast you can go. But Earthlings don't have the convenience of utter vacuum, because the stuff they breathe always gets in the way. Air is made up primarily of molecules of nitrogen, carbon dioxide and nitrogen, which can be compressed, decompressed, squeezed and shoved out of the way. But those molecules will always try to return back to their original positions. This makes air an "elastic" substance, like a progressive spring. The harder you push against it, the harder it pushes back. Eventually, at some speed, even the most powerful vehicles on Earth will lose this fight with air resistance; the only question is how fast they can go given the power they have.

Mathematically, air resistance increases as a square of itself. As a substance, air effectively gets "thicker" with speed. At 30 mph, overcoming air resistance is like driving through whipped cream. At 100 mph, it's like driving through water. At 200 mph, the air is as "thick" as cold molasses, and getting to 300 mph is like trying to shove a Matchbox car through a four-year-old fruitcake. The rule of thumb for speed and air resistance is that for a given car, it takes four times as much power to go twice as fast. So, if your car can do 50 mph with 35 horsepower, you'll need 140 horsepower to do 100 mph. For 200 mph, you'll need about 560 horsepower. At 300 mph, it would be around 2,240 horsepower. To break the sound barrier, you'd need around 10,000 horsepower -- roughly the equivalent of a Top Fuel engine.

The squaring rule of thumb only applies if your car is "power limited," or limited in speed only by the amount of power it produces. In the real world, most stock vehicles are electronically limited. In the United States, where the government has yet to institute a maximum governed speed, factory speed limitations usually have more to do with the stock tires' maximum speed rating than anything else. Other countries like Japan -- 112 mph -- institute a blanket electronic speed limit. Many European cars are limited to 155 mph, a kind of "gentlemen's agreement" among manufacturers to prevent an all-out speed war on Germany's Autobahn. The Autobahn has long been a source of controversy in Germany, and it was thought at the time that a speed war and the inevitable 300 mph crashes would sway the public to impose a blanket speed limit on the Autobahn.

Cars that aren't either power-limited or electronically limited are said to be "gearing limited." Gearing limitations means that the car can't reach its theoretical power-limited top speed either because the gearing is too low -- causing the engine to hit redline before top speed -- or too high, which keeps the engine from reaching its peak power rpm. For most engines, horsepower rises to a peak near the engine's maximum safe rpm, or "redline," and then begins to drop off after the peak. A car that's geared very low, like one designed for the quarter-mile, will accelerate very quickly but will run out of engine rpm before it hits its theoretical top speed. Such cars might alternatively be considered "rpm limited." Other cars have gearing that's too high, causing air resistance at a certain speed to overcome the engine's power before it reaches its peak horsepower rpm. This is why many gearing-limited supercars will see a higher top speed in fifth gear than they would in sixth.

As of 2014, the fastest stock vehicles on Earth run between 250 and 270 mph -- but there's no reason they or you couldn't go faster. Assuming your car isn't limited by gearing or an electronic governor, you've got two options for going faster: more power or less aerodynamic drag. All else being equal, reducing drag is usually the cheapest and most efficient way to increase top speed. Ideally, you'd want your car to be low, as short and small as possible when seen from the front, and pointed at both ends with a wedge-shaped nose and a "boat tail" rear. Any gaps in the sheetmetal, protrusions like mirrors, door handles and wings and open grilles in the front of the car will slow you down. Most top-speed racers use a low "air dam" on the front bumper to keep air from going under the car, and a smooth sheetmetal "belly pan" to help any air that does make it below get out. This isn't just to reduce drag -- high-pressure air building up under the car's body can cause it to lift like a wing and take flight at high speed.