When driving, everyone pays excellent attention to speed, especially considering the abundance of speed-limited areas nowadays.

A slight oversight could lead to a fine. Additionally, there's a lingering curiosity in everyone's mind: if there were no speed limits, how fast could a car go?

Looking at the speedometer, you might see maximum values like 240, 280, or even 330. But does that mean the car can genuinely reach those speeds? Why do some vehicles have high top speeds, like Formula 1 race cars, while others have significantly lower top speeds, like large trucks? Let's delve into the mysteries behind automotive speed.

First, let's define speed. In physics, speed is defined as the magnitude of distance an object travels in a unit of time. However, this definition is not precise, as speed has magnitude and direction; it's a vector quantity. What we're more concerned with for cars is instantaneous speed, the value displayed on the car's speedometer, which holds the most significance for us.

So, how is a car's speed determined? In earlier car models, there was a speed sensor on the transmission's output shaft, measuring its rotational speed. The car's speed could be calculated and displayed on the dashboard by considering parameters like the car's primary reduction ratio and wheel radius. This method is still used in many trucks.

Most modern passenger cars now use wheel speed sensors in the ABS to sense the rotation speed of the wheels. These sensors send signals to the car's computer. After processing through preset programs, the computer calculates a value displayed on the dashboard as the vehicle speed.

Of course, this displayed speed comes with some margin of error. Typically, the indicated speed on the dashboard tends to be slightly higher than the actual speed. According to the "Speedometer for Motor Vehicles" standard (GB15082-2008), the indicated speed should not be lower than the actual speed. The relationship between the predicted and actual speeds should meet specific criteria.

But how fast can a car go? Some people with good mathematical skills calculate a value based on various car parameters. For instance, if the engine's maximum speed is 5000 revolutions per minute, with a top gear ratio of 1, a primary reduction ratio of 4, and a wheel radius of 0.4 meters, the calculated maximum speed is 188 kilometers per hour. However, the car may only reach 120 kilometers per hour. Why is that?

It turns out that when a car travels at high speeds, it encounters various forms of resistance, such as rolling, acceleration, incline, and air resistance. The car reaches its top speed when the engine outputs maximum power and balances with the driving resistance.

Therefore, a car's top speed is mainly influenced by three factors:

1. Maximum Engine Power:

The engine's maximum power is the decisive factor determining a car's top speed. As the saying goes, "Acceleration depends on torque, while top speed depends on power." If the engine has low power output compared to the vehicle's weight, the car's top speed will be limited. Only when the engine is outputting maximum power, and the car is traveling at a constant speed will the speed be at its maximum.

2. Air Resistance at High Speeds:

Among various driving resistances, air resistance significantly impacts a car. Tests show that when a car exceeds 120 kilometers per hour, air resistance accounts for about 80% of the total driving resistance.

When the speed surpasses 180 kilometers per hour, nearly all resistance comes from air resistance, rendering other resistances negligible. When a car travels at its maximum speed, when the driving force balances with the primarily air-based driving resistance, a more minor air resistance allows for a higher top speed. Hence, the aerodynamic coefficient affects a car's maximum speed.

3. Total Transmission Ratio of the Car's Transmission System:

The transmission ratio of a car's transmission system restricts the ratio between the engine speed and the wheel speed, thereby limiting the maximum wheel speed and, consequently, the car's top speed.

For instance, a tractor has a high transmission ratio, so its top speed remains limited even with powerful engines. On the other hand, passenger cars typically have lower transmission ratios, resulting in higher wheel speeds and thus achieving higher speeds. This explains why trucks, despite having high power, have lower speeds.

A car's top speed is a benchmark for assessing its performance, primarily determined by the engine's power. However, greater engine power does not necessarily equate to higher top speeds; it's also constrained by driving resistance and transmission ratios. When a car's power and resistance reach equilibrium, it can achieve its maximum speed.

While a vehicle's power could theoretically allow it to reach its maximum speed, there are limitations to engine power and transmission ratios. Hence, a car's top speed results from manufacturers tuning it based on multiple factors, often serving as a reference rather than an absolute value.