The idea
Beginner explanation — switch to Technical for the engineering detail.
A moving car has to shove a huge amount of air out of its way. At walking pace that is trivial, but the force grows with the square of speed: going twice as fast means four times the air resistance. On the motorway, air resistance usually takes more of the engine's effort than anything else.
Two things decide how much drag a car makes. One is its size seen from the front (frontal area) — a tall van pushes more air than a low saloon. The other is how cleanly it lets the air flow around and close up behind it, measured by the drag coefficient. Smooth shapes, gently tapering tails and covered underbodies let the air rejoin behind the car with less turbulence.
Air also pushes up or down on a car. Some shapes generate lift at speed, which makes the steering feel light. Spoilers, wings and shaped undersides can reduce lift or even press the car down (downforce) for more grip — but usually at the cost of extra drag.
What you will learn
- Use the drag equation to estimate aerodynamic force and power
- Explain why drag power rises with the cube of speed
- Distinguish drag coefficient from frontal area and how each is reduced
- Explain lift, downforce and the role of spoilers, diffusers and underbody panels
How it flows
Read the diagram as text
- Road speed → Drag force
- Shape & size → Drag force
- Drag force → Engine / motor power (∝ v³)
- Road speed → Lift / downforce
- Shape & size → Lift / downforce
- Road speed: q = ½ ρ v²
- Shape & size: C_d × A
- Drag force: F = q C_d A
- Engine / motor power: P = F v
- Lift / downforce: Changes tyre load
Step by step
Read each step in order.
Step 1: Pushing air aside
A moving car must push air out of the way, and the faster it goes the harder that becomes.
Step 2: The drag equation
Drag depends on speed, the car's size from the front and how smooth its shape is.
Step 3: Power rises with the cube
Going a little faster costs a lot more energy: double the speed takes eight times the power to beat air resistance.
Step 4: Shape the wake
Smooth shapes and tidy tails let air close up gently behind the car, making less turbulence.
Step 5: Manage lift
Spoilers and shaped undersides stop the car feeling light at speed, or press it down for grip.
Go deeper
Optional. Open a panel for the engineering detail — or switch to Technical above.
The engineering explanation3
Aerodynamic drag is F_d = ½ ρ v² C_d A, where ρ is air density (≈ 1.2 kg/m³ at sea level), v the speed relative to the air, C_d the drag coefficient and A the frontal area. The product C_d A (drag area) is what really matters: a modern saloon might have C_d ≈ 0.25–0.30 and A ≈ 2.2 m², while an SUV may have both a higher C_d and a larger area. Because power is force times speed, drag power is P = ½ ρ C_d A v³. For C_d = 0.30 and A = 2.2 m² at 120 km/h (33.3 m/s) the drag force is about 450 N and the power about 15 kW — roughly three times the rolling resistance of a 1,500 kg car with a rolling-resistance coefficient of 0.01. Doubling speed raises drag power eightfold.
Most passenger-car drag is pressure drag from the low-pressure wake behind the car, not skin friction. Designers reduce it by delaying flow separation and shrinking the wake: rounded front corners, a gently sloping or tapered tail ending in a sharp cut-off (the Kammback principle), tight panel gaps, flush glazing, wheel-arch air curtains and smooth underbody trays. Cooling airflow through the radiator and engine bay adds drag, so active grille shutters close when cooling demand is low. Mirrors, wheels and tyres contribute disproportionately.
Lift arises from pressure differences between the upper and lower surfaces. A rear spoiler trips the flow to reduce rear lift; a wing generates downforce like an inverted aerofoil; a rear diffuser expands the underbody flow gradually, lowering pressure under the car to create downforce with relatively little drag. Downforce raises tyre normal load and therefore grip without adding mass, but the associated induced drag costs efficiency. Crosswind stability depends on the relationship between the centre of pressure and the centre of gravity.
Key concepts4
- Drag force
- Aerodynamic resistance opposing motion.
F_d = ½ ρ v² C_d A- Drag power
- Power needed to overcome drag; it grows with the cube of speed.
P_d = ½ ρ C_d A v³- Drag coefficient (C_d)
- A dimensionless measure of how efficiently a shape moves through air, independent of size.
- Downforce
- Negative lift that pushes the tyres onto the road, increasing grip without adding mass.
Common misconceptions3
MythA low drag coefficient means a car has low drag.
RealityDrag depends on C_d multiplied by frontal area. A large vehicle with a good C_d can still have more drag than a small car with a worse one.
MythA rear wing on a road car always improves performance.
RealityWings add downforce but also drag. Poorly designed add-ons often increase drag and fuel use with no useful grip at legal speeds.
MythOpening the windows is always more efficient than using the air conditioning.
RealityAt low speeds it often is, but at motorway speeds open windows can add enough drag to rival the A/C's energy use.
In the real world3
- Driving at 110 km/h instead of 130 km/h cuts the power needed to overcome drag by around 40%.
- A roof box can raise motorway fuel consumption noticeably because it increases both frontal area and turbulence.
- Electric cars often use smooth aero wheel covers and flat underbodies because range is so sensitive to drag.
Common questions
Short answers to the questions people most often ask about this topic.
What is a drag coefficient?
It is a number that describes how easily a shape moves through air, independent of its size. Drag force depends on the drag coefficient multiplied by the frontal area, the air density and the square of speed, so a car's total drag needs both numbers.
Why does fuel use rise so sharply at high speed?
Aerodynamic drag force grows with the square of speed, and the power needed to overcome it grows with the cube. Going from 100 to 130 km/h therefore needs roughly twice the power just to push through the air.
What is downforce?
Downforce is aerodynamic force pressing a car onto the road, produced by wings, diffusers and shaped underbodies. It increases tyre grip without adding weight but usually adds drag, which is why it matters most on racing cars.
Check your understanding
3 questions. Your best score is saved in this browser.
0 of 3 answered
Question 1 of 3
If speed doubles, what happens to the power needed to overcome aerodynamic drag?
The story behind it
Who worked this out, and what they were up against. Optional.
1938
Wunibald Kamm
Automotive aerodynamicist, Stuttgart
Showed that a chopped-off tail can behave like a long one.
The Kammback is a car body style based on Wunibald Kamm's research into reducing turbulence at high speed: keep a smooth roofline and taper, then cut the tail off abruptly. The airflow behaves almost as if the full tail were still there, so drag stays low without a long tapering body. In late summer 1938 BMW tested a 'Kamm-Coupé' prototype with a drag coefficient of 0.25, while the 1940 Mille Miglia-winning BMW 328 Touring Coupé measured 0.35. The Stuttgart research institute FKFS, established in 1930, was nicknamed the Kamm-Institut.
Sources
Retold in our own words from the sources above.
Almost there
- Overview — Read the introduction to the end.
- See it work — 5 steps left to read
- Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
- Check — Answer the quiz questions.
- Story (optional) — Optional: who worked this out?
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Sources & attribution
Further reading
- Car aerodynamicsHow a Car Works · consulted 2026-10-21(external site, opens in a new tab)
Further reading on an external site. Linked only; AutoVerse lesson text is written independently.
- Licence
- AutoVerse original content
This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text.