One of the many factors that affect your fuel economy is the ride height of your vehicle. In general, the lower your car is, the better your fuel economy will be. This is due to a few different factors. We will cover the basics of why this is the case here.
The number one use of your fuel at highway speeds is to overcome wind resistance. At speeds over 60MPH more than 50% of the fuel you burn is due to air drag. This is true of almost all production cars. Some vehicles such as large trucks and SUVs are actually worse than this. Some small cars like the Prius and the Insight are not as bad as this.
For a better understanding of how aero drag uses so much fuel, think back to when you were a kid. Remember holding your arm out of the car window? When you hold your hand like an airplane wing, it cuts through the air smoothly. When you face your palm directly at the air, it forces your whole arm back quickly. If you turn your hand sideways again, the force is much less and your arm moves back to its original spot with very little resistance. This is exactly how it works with a car, except that your car is either shaped like a flat hand, an open hand, or somewhere in between, and can’t be easily changed.
Speed is the determining factor in how much aerodynamic drag you create. For a simple example of this, think of what it’s like to move your arm under water at the pool. If you move your arm slowly you can move easily and have almost no resistance. If you try to move your arm fast, you create a lot of resistance for yourself. The faster you move, the stronger the resistance is. The same thing is true of automobiles. At 5 MPH, there is basically 0 aerodynamic drag. At 40 MPH there is some aero drag. At 150 MPH there is enormous aero drag. The main determining factor of a cars top speed is the amount of drag it creates, not its engine size or horsepower. Most vehicles get to a speed where the drag is too strong for the engine to overcome and that is the top speed. Lower the drag a little and the vehicle now has a higher top speed.
The amount of resistance your car has to the air is called “aerodynamic drag”. Every car is rated for its aerodynamic drag and given a number called a CD number. CD stands for coefficient of drag. The lower the CD number a vehicle has, the smoother it goes through the air. The higher a CD number that a car has, the more difficult it is to push through the air. A high CD number vehicle is a vehicle that will use a lot of fuel at speeds above 60 MPH to overcome its poor aerodynamics. To give you a basis for comparison, a Prius has a CD number of 0.25. A Hummer H2 has a CD number of 0.57. This is pretty much the CD range that production vehicles fall between.
So how do you lower your CD number?
Well, there aren’t many really easy things you can do to lower your CD. The basic shape and lines of your car or truck were designed by the manufacturer. The way the air flows over and around your car is something that would take major work to change significantly.
So why are we talking about this?
There’s another number called the CdA of a vehicle. That number takes your cars CD and multiplies it by its frontal area. The CdA is called your “drag area”. This drag area is used by a lot of places as a more accurate measurement of how aerodynamic your vehicle is.
Here’s an easy way to think of drag area. Imagine two balls. One ball is big and one ball is small. Both of the balls will have the same CD number because they both have the same shape. The CD number for a sphere is 0.47. All spheres have a CD of 0.47. A marble and the earth both have a CD of 0.47. So if you just look at CD numbers, it would appear that both of these balls would have the same aerodynamic drag. It would appear that both balls would require the same amount of power to overcome wind resistance. We know this isn’t true though because one ball is bigger than the other. The bigger ball has more frontal area that will be catching air as it moves. The same is true of cars. You can have two cars with the same CD number, but if one is wider and taller than the other, it creates more drag.
So multiplying the CD by the frontal area of your car gives you your CdA. If you have two cars with the same CdA number, they will truly have the same amount of aero drag. These two cars may be different sizes and different shapes, but if the CdA is the same, they are aerodynamically identical as far as drag goes.
Lowering your ride height reduces your frontal area.
The easiest way to lower your CdA is to decrease your frontal area. Frontal area is the area of your car that is directly impacting the oncoming air. If you stand in front of your vehicle and look directly at it, what you see is the frontal area. If your car is 4’ tall and 5’ wide, your frontal area is 20 square feet. If your CD number is 0.30, your CdA is 6.0. If you lower your ride height you decrease your frontal area. Decreasing your frontal area lowers your CdA and aerodynamic drag. Lowering your drag improves fuel economy and top speed.
Lowering your ride height improves airflow under the car too.
Many racecars use a front air dam. A front air dam is a device that goes on the bottom front of the car to block air from going under the vehicle. This helps a lot with the stability of the vehicle at high speeds and reduces overall drag. Lowering your vehicle has the same effect as installing an air dam. Lowering allows less air to get under your car and disrupt the handling at speed.
The lower your vehicle is, the better it handles.
A car that has a ride height that is too high will handle terribly. Lowering your ride height will lower your center of gravity. A lower center of gravity will help your tires stick to the ground in hard corners. A lower center of gravity will also decrease body roll in corners and during hard braking.
To sum all of this up, there are many benefits and very few drawbacks to lowering your ride height. Today it is easy to buy an aftermarket kit, or have a shop adjust your height. It’s been estimated that lowering your ride height by 1-2” gives you 10% better fuel economy during highway driving. This is true because it lowers your drag by reducing your frontal area. Now you know how and why that works.