How top speed is calculated
At top speed all the wheel power is spent pushing air and rolling the tires:
P = ½ × ρ × Cd × A × v³ + Crr × m × g × v
The aero term grows with the cube of speed — double the speed and drag power is eight times higher. That's the aero wall: gaining the last 10% of top speed needs roughly 30% more power. This tool assumes ~15% drivetrain loss, standard air (1.225 kg/m³) and a typical rolling-resistance coefficient, and that gearing lets the engine reach the drag-limited speed.
FAQ
Why doesn't weight matter much?
Weight sets rolling resistance, which is a small share of the total at high speed — aero dominates. Weight hurts acceleration far more than top speed, which is why heavy-but-slippery cars still post big vmax numbers.
Where do I find Cd and frontal area?
Cd is usually published (0.28–0.36 for most cars). Frontal area is rarer: approximate it as ~0.84 × width × height, or use ~2.0–2.3 m² for a coupe/hatch and 2.5–3.0 m² for an SUV.
Could gearing limit me first?
Yes — many cars hit redline in top gear before the drag limit, or are electronically limited. Cross-check with the Gearbox Designer: your true vmax is the lower of the two.