what determines how fast a car can go

[VonAlexa]

The top speed of a car is primarily determined by the balance between the power its engine produces and the resistance it must overcome, mainly from aerodynamics and rolling friction. When the engine can no longer produce enough power to push against the increasing air and tire resistance, the car reaches its maximum velocity. While a powerful engine is crucial, it's not the only factor; a car with massive horsepower but poor aerodynamics will be limited compared to a more streamlined vehicle.

The most significant resistance at high speeds is aerodynamic drag. This force increases with the square of the speed; meaning, if you double your speed, the drag force quadruples. A car's drag coefficient (Cd) is a measure of its aerodynamic efficiency. A lower Cd value means the car slips through the air more easily, requiring less power to achieve a high speed. This is why supercars like the Tesla Model S Plaid (Cd 0.208) can achieve remarkable top speeds with less power than some competitors.

The powertrain delivers the energy. Horsepower is the key metric for top speed, as it represents the engine's ability to sustain work over time. The transmission's gearing is also critical; the final drive ratio must be calibrated to allow the engine to reach its peak power RPM at the desired top speed. If the gearing is too short, the engine will hit its rev limiter prematurely. If it's too tall, the engine may not have enough power to overcome the resistance.

Other factors include weight (which affects acceleration but less so top speed on a flat surface), tire type and pressure (influencing rolling resistance), and even environmental conditions like air density and wind.

Factor	Description	Impact on Top Speed	Example Data (Various Models)
Engine Power	Peak horsepower output.	Directly proportional; more power generally enables higher speed.	Dodge Challenger SRT Hellcat: 717 hp / 196 mph
Aerodynamics	Drag coefficient (Cd) and frontal area.	Critical; lower drag allows higher speed with same power.	Tesla Model S Plaid: Cd 0.208 / 200 mph
Transmission Gearing	Final drive ratio and number of gears.	Determines if engine can reach power peak at terminal velocity.	Bugatti Chiron: 7-speed dual-clutch, long gearing for 261 mph
Vehicle Weight	Curb weight of the vehicle.	Minor impact on ultimate top speed; major impact on acceleration.	Lotus Exige: ~2,400 lbs / 145 mph
Tire Rating	Speed rating (e.g., V, W, Y).	A physical limit; tires must be certified for the target speed.	Y-rated tires are certified for speeds over 186 mph.
Drivetrain Losses	Power lost through transmission, differential.	Reduces effective power at the wheels.	All-wheel drive systems typically have higher losses than RWD.

[PresleyLee]

Think of it like pushing a brick through water. The engine is your muscles. A bigger, stronger engine helps, for sure. But the real battle is against the air. Once you get past 60 or 70 mph, the air feels like thick syrup. A car designed to slice through that syrup—like a slippery sports car—will always go faster than a boxy truck with the same engine. It's all about the shape and the power working together.

[BenjaminDella]

It's an engineering equation. Horsepower pushes the car forward, while aerodynamics and tire friction hold it back. Top speed is the point where these forces are perfectly balanced. The transmission's job is to make sure the engine is spinning at its most powerful RPM right when that balance happens. So, you can't just drop a huge engine in any car and expect a record; every part of the drivetrain and body has to be optimized for that single goal.

[SanCamila]

Most folks just think about horsepower, and that's the biggest piece. But I've learned it's the details that matter. The gearing in the transmission has to be just right—if the top gear is too tall, the car will feel sluggish. Then there's the tires; they have to be rated for the speed and inflated correctly to minimize rolling resistance. Even the weather plays a role. A cool, dense day is better for top speed runs than a hot, humid one because the engine breathes better and the air is slightly thinner.

[StellaDella]

From my perspective, it's a tug-of-war. On one end of the rope, you have the engine's horsepower and the gearing that multiplies it. On the other end, you have two main opponents: air resistance and tire friction. The winner of this battle determines the top speed. Air resistance becomes the dominant enemy at high speeds, which is why supercars have such wild, wind-cheating shapes. It's not just about raw power; it's about engineering efficiency to minimize the forces working against you. A car with 700 horsepower but the aerodynamics of a barn door will lose to a 500-horsepower car with a sleek, low-drag design.