Aerodynamics affects a car’s top speed and fuel economy, among other metrics, and so car manufacturers try to make their vehicles as aerodynamically “slippery” as possible. Slippery cars require less effort to cut through the air, and less effort means reduced fuel usage — which is a good thing.
However, regardless of how aerodynamic a design is, the laws of physics say aerodynamicists can only do so much. Ultimately, friction is unavoidable, and all we can do is minimize its influence on fuel efficiency, not eliminate it. Unfortunately, this means that even a slight wind can play havoc with your mpg. For instance, driving into a 5-mph headwind increases your fuel usage by just over 10%.
Understanding this takes a simple bit of arithmetic. We’ll need to know the vehicle’s speed, wind speed, and have a baseline MPG figure. Note that these account for roughly half the total resistance acting on a vehicle, with rolling resistance from the tires, drivetrain losses, and other factors making up the remainder. While we will focus on the aerodynamics here, we’ll need to account for this in the calculation. Calculators at the ready!
How wind affects fuel economy
There are good mechanical reasons why your vehicle’s mpg is going down, but wind has a bigger effect than you may imagine. Much of this can be attributed to one fact – aerodynamic drag increases with the square of speed.
Bearing that in mind, we can get on with the calculation. Let’s start by assuming you’re doing 50 mph on a good highway. Now, let’s add a 5-mph headwind to the picture and see what happens. We can add the two figures to get a total airspeed of 55 mph. However, since we’ve already established that aerodynamic drag increases by the square of speed, this 5 mph wind will punch above its weight. Finally, we need to add a baseline MPG figure. According to the US Department of Energy, the average mpg of US cars is about 25; we’ll roll with that.
Firstly, we can see what effect an extra 5 mph has on aerodynamic drag. We do this by squaring both numbers. At 50 mph, we get 2,500; at 55 mph, this rises to 3,025. Dividing the two figures shows that the aerodynamic drag is now 1.21 times, or 21%, higher than before. However, because aerodynamic drag accounts for about 50% of total resistance, only half of that increase carries through to the overall figure, for a total increase of about 10.5%.
Essentially, this means you’re using 10.5% more fuel in a 5-mph headwind. As an example, then, a 100-mile, wind-free journey that consumed four gallons at 25 mpg would use 4.42 gallons with a 5-mph breeze, dropping the fuel efficiency to 22.62 mpg (100/4.42).
Stephan is the sports journalist for the Maple Grove Report.
