What determines a car's top speed?

A car's top speed is determined by the balance between how much power the engine produces and how much resistance the car has to overcome at higher speeds — primarily aerodynamic drag, but also gearing and rolling resistance.

The main factors:

• Power output — more power gives the engine more capacity to overcome resistance at higher speeds
• Aerodynamic drag — as speed increases, air resistance increases significantly (roughly with the square of speed), so a car's shape and drag coefficient play a major role in how much power is needed to keep accelerating at higher speeds
• Gearing — the final drive ratio and top gear ratio determine how the engine's power translates into road speed; a car geared very short for acceleration may hit its engine's limit before reaching its theoretical top speed
• Weight and rolling resistance — heavier cars and higher rolling resistance (from tyres, for example) also require more power to maintain higher speeds

Why manufacturer figures can vary from real-world results:

Manufacturer-quoted top speeds are typically achieved under controlled test conditions, and some vehicles are also electronically limited well below their theoretical maximum for safety, tyre-rating, or regulatory reasons — a limitation that's separate from what the engine and aerodynamics alone could achieve. If you're comparing top speed figures between two cars, it's worth noting whether either is electronically limited, since that can make the raw figures less directly comparable to the vehicle's actual mechanical potential.