Most people think figuring out the aerodynamic drag on their vehicle requires a $50,000 facility and a team of engineers. It doesn’t. You can actually calculate how much drag is on your car using basic physics, a stopwatch, and a friend. It’s surprisingly simple if you strip away the corporate jargon and stick to Newton’s second law.
The core concept is straightforward. Newton’s second law states that Force equals mass times acceleration (F = ma ). When you cruise at a steady speed, the engine’s power translates to force at the tires. At that exact moment, the aerodynamic drag pushing back is equal to the force pushing forward. The net force is zero. You aren’t accelerating. You aren’t slowing down. You are in equilibrium.
Put the car in neutral, and that balance vanishes. The engine stops pulling. Gravity and rolling resistance take over, but the dominant force slowing you down is air resistance. By measuring how fast you decelerate, you can work backward to find the drag force.
The Road Load Equation
Before you run out onto the highway, you need to understand the forces at play. The total resistance to motion—often called road load force—follows a specific formula:
road load force = a + bv + cv²
Here, v is your velocity. The letters a, b, and c are constants that represent different physical resistances:
- The ‘a’ term: This is speed-independent. It comes from rolling resistance in the tires and internal friction, like brake pads rubbing slightly or wheel bearing drag.
- The ‘b’ term: This also stems from component friction and rolling resistance, though it scales slightly with speed.
- The ‘c’ term: This is the big one for enthusiasts. It represents aerodynamic drag. It depends on your frontal area, your drag coefficient (Cd), and air density.
Notice the v² in the equation. The force doesn’t grow linearly; it grows exponentially. The drag at 70 mph is significantly higher than at 60 mph. This is why high-speed efficiency matters so much.
Because the force changes rapidly with speed, you need to measure acceleration over a very narrow window. A span of 3 mph or 5 kph is ideal. We’ll use metric units for the math, as they are cleaner.
The Experiment
Let’s say your car, plus you and your co-pilot, weighs 2,000 kilograms. You want to test the drag at around 60 mph. In metric, that’s roughly 97.5 kph. So, you need to measure deceleration between 100 kph and 95 kph.
Find a long, flat stretch of highway. Low traffic is essential. Pick a day with calm winds and dry pavement. Rain adds weight and changes tire grip; wind introduces variables you can’t control.
Have your passenger drive up to 105 kph. Switch to neutral. Start coasting. When the speedometer hits 100 kph, start the timer. Stop it when the car drops to 95 kph. Do this a few times, maybe driving in the opposite direction to average out any slight inclines or wind gusts. Record the times.
Let’s assume your average time is 10 seconds. Now, do the math.
First, convert the speed drop to meters per second. A 5 kph drop is 5,000 meters per hour. Divide by 3,600 to get seconds. That’s 1.389 meters per second.
If it took 10 seconds to drop that speed, the deceleration is:
1.389 m/s ÷ 10 s = 0.1389 m/s²
Now, plug this into F = ma.
Mass = 2,000 kg
Acceleration = 0.1389 m/s²
Force = 2,000 × 0.1389 = 277.8 Newtons
To put that in imperial terms (since most American enthusiasts think in pounds), 277.8 Newtons is roughly 60 pounds of force. This means at 60 mph, your car is fighting against 60 pounds of wind resistance. To maintain that speed, your engine must generate 60 pounds of force at the wheels.
Calculating the Horsepower Loss
Knowing the force is useful, but most people want to know the power loss. Power is simply Force multiplied by Speed.
Use your average speed for the test: 97.5 kph, which is 27 meters per second.
Power (Watts) = Force (Newtons) × Speed (m/s)
Power = 278 N × 27 m/s = 7,500 Watts
Convert that to kilowatts, and you get 7.5 kW. In horsepower, that’s roughly 10 HP.
So, at 60 mph, just to overcome aerodynamic drag and rolling resistance, your car is using about 10 horsepower. The rest of your engine’s output goes to things like the alternator, AC compressor, and overcoming internal friction.
It’s a small number, but it adds up. At 100 mph, that drag force triples, and the power required to overcome it increases dramatically. This is why a sleek sedan gets better highway mileage than a boxy SUV, even if they have the same engine. The air doesn’t care about your brand loyalty. It only cares about shape and speed.
Measure it once. You might be surprised how much energy is just… gone. Into the wind.
