Why the aero wheels on the Tesla Model 3? Does it really help as much as claimed? Why don't we see these on ICE cars?
When it was first disclosed that the Tesla Model 3 Aero wheel covers were potentially giving a 10% range/efficiency gain, I started thinking about why this was not important on all cars. If you could get a 10% mileage gain, why wouldn’t all cars have this type of wheel cover? The reality is that the impact on an Internal Combustion Engine (ICE) car is minimal. On an Electric Vehicle (EV) aerodynamics play a much bigger role.
The starting point to understanding this is comparing the energy consumption of an ICE car to a typical EV. For illustration purposes, I have chosen the Ford Focus EV. The chart below shows the energy consumed in kWh for a car traveling at highway speeds for 60 miles. As most of us know, the EV consumes much less energy. Approximately 21 kWh for the EV versus 60 kWh for the ICE.
These have been calculated using the highway MPG/MPGe estimated from the EPA site for the Ford Focus using 33.7 kWh equivalent for one gallon of gasoline. This difference is an important point to consider for anyone arguing that an EV is just pushing the emissions point to a plant with a smokestack. The starting consumption point is so much lower for an EV, that in almost every case, it emits less overall emissions than an ICE car.
Now, we need to show where the energy is consumed. Rolling resistance is mostly caused by the tires and wheels and is most influenced by the weight of the vehicle. The wind resistance is all related to aerodynamics. We use the same absolute losses for the EV for wind resistance, rolling resistance, lights, fans, etc. This is one reason I used the Ford Focus for comparison.
The Focus ICE and EV versions are almost identical, so you would not expect the aerodynamic or rolling resistance losses to be significantly different. The dramatic change is for the powertrain losses, which have dropped from 45 kWh ICE to 6 kWh for the EV.
The following chart breaks down where the energy is consumed in this example for the ICE and EV, while keeping the wind, rolling, fans, and light losses the same between the EV and ICE. Again, we're assuming highway speeds for one hour. These are probably not 100% precise for the Ford Focus but are based on typical averages which are directionally correct for this example.
Now, we need to show the losses as a percentage of the total for each vehicle. If you are trying to improve the efficiency of a particular vehicle, this chart shows you the focus areas (no pun intended). For the ICE, it's clear the engine/drivetrain is the biggest area of loss and the area where the most gain is possible. The profile of the loss changes dramatically for an EV, with the wind resistance and rolling resistance taking a much larger percentage of the total at highway speeds.
For the ICE, 75% of the losses are in the engine/drivetrain. If you can engineer a 10% improvement in the engine/drivetrain, the vehicle will get 7.5% (75% of total losses x 10% improvement) more efficient. The reality is getting a 10% efficiency gain in the powertrain of an ICE vehicle is difficult. This also explains why there is a large amount of cost and complexity in today’s ICE vehicles with turbos, variable timing, etc.
Additionally, a 10% improvement in the aerodynamics of an ICE vehicle gets very little efficiency improvement on the highway. It's 15% of the total losses and a 10% improvement would only yield a 1.5% improvement in vehicle efficiency. This explains why few ICE vehicles rely on aero type wheel covers.
For the EV, the wind resistance becomes the largest loss on the highway at somewhere north of 40% of the total losses. If you can improve the aerodynamics 10%, the range and MPGe will increase by 4% (40% of the total x 10% efficiency improvement).
In fact, I believe it's higher than this as the drivetrain losses are likely overstated in this example at 25%. Typical drivetrain losses for an EV are less than 20%. In addition, rolling resistance becomes more of a factor as well, which explains why most EVs have low rolling resistance tires.
So, it's likely that the Aero covers on the Tesla Model 3 improve the aerodynamics about 10%, which leads to a 4% overall efficiency improvement. This is in line with various testing posted online by Model 3 owners. The quoted 10% total efficiency difference is likely comparing the 18” wheels with the aero covers to the 19” sport wheels, which also likely increase the rolling resistance.
It will be interesting in the coming years to see some of the innovations in aerodynamics and tires to further reduce the losses in these areas. Smaller losses in aerodynamics and rolling resistance can allow longer range and/or a battery size reduction. For example, the idea to remove the side-view mirror and use cameras will also benefit an EV much more than an ICE car.