Electric Cars Don’t Use Gas, So What Do We Do About MPG?
Internal combustion cars seem to have been around forever. OK, it hasn’t been forever. At least by 1910, cars that ran on gasoline were quite common. I suspect that everyone reading this was born in the automobile era. That means that we have all had the experience of filling a car at the gas station. We know that gas goes into the tank and the car consumes this fuel to go vroom, vroom and get you to where you need to go. With all of this gasoline experience, we all have a pretty good feel for fuel efficiency. In the USA, we know that a car that gets 15 miles per gallon (mpg) is not very efficient. If you get 40 mpg, that’s pretty good.
But what about an electric vehicle (EV)? In case you don’t know, these things don’t use gasoline (at least not directly). Instead, they use a rechargeable battery to get you to the grocery store and back. But still, all EVs are not equal. Some of them have larger batteries (more energy) and then some of them have less stored energy but also use less energy. Humans want a single number similar to the mpg for these electric cars, but what do we use? Let’s look at some options and see if we can all agree with a way to compare electric vehicles to both other EVs as well as gasoline burning cars.
It’s All About Energy
Gasoline or a battery — it doesn’t matter, they both store energy. In physics, we find energy as a very useful way to explain interactions. In general, when the energy stored in the car decreases that energy goes into some other form. The first type of energy we should think about is the energy of motion. When an object is moving, it has kinetic energy (K) that depends on the mass of the object as well as the speed.
If the mass is in units of kilograms and the velocity in meters per second, the kinetic energy will be measured in Joules (there are other units for energy, but this one is the best). Now imagine you have your nice electric car with a mass of 1500 kilograms. It starts off at rest (with a velocity of 0 m/s) and then increases in speed to 20 m/s. This means that there is an increase in kinetic energy in an amount of 30,000 Joules to speed the car up. When the car stops, there would be a decrease in kinetic energy. For internal combustion vehicles, this decrease in energy during a stopping motion just goes into heating up the brake rotors — so, it’s basically just thrown away. With an electric car, they can use regenerative braking. In this process, the electric motor that normally drives the wheel is turned into an electric generator to charge the battery (somewhat). Of course the process isn’t 100 percent efficient so that you still lose energy (to heat).
What about a car driving along at a constant speed? Even though there isn’t a change in kinetic energy, this also requires some energy from the battery or gas tank. It’s all about air resistance. Imagine a car driving along at a constant speed. We can then draw the following force diagram for this situation.
There’s of course the downward pulling gravitational force along with the upward force from the ground (we call it the normal force since it’s perpendicular to the surface). These two forces have equal magnitudes such that the net force in the vertical direction is zero and the car stays on the ground (that’s probably what you want). In the horizontal direction there is a backwards pushing air resistance force. In order to move the car at a constant speed, there needs to be a frictional force between the tires and road to make the net force zero. However, with this frictional force it takes energy to rotate the tires and this energy also comes from the battery or gasoline tank.
So, clearly energy is important. In fact, we could even say that gas mileage is a measure of energy per distance. Let’s introduce a new idea (it will be useful) — the energy density. This is the amount of stored energy per unit volume. Gasoline has 34.2 million Joules in every liter of fuel. Notice that in just one liter, there’s a WHOLE BUNCH of energy in that gasoline. This is why it’s quite difficult to switch away from burning fossil fuels. Of course fossil fuels also have a pretty big problem — that whole carbon dioxide and climate change things. That’s bad.
But how much energy do you get from 1 gallon of gas? I just need to convert units from liters to gallons. Also, let’s use an efficiency of 30 percent since not all of the energy in the fuel goes into the motion of the car. This means that you can get 38.9 million Joules for every gallon of gas. Nice.
Energy per Mile
Since we already think about efficiency in miles per gallon, something like energy per mile (or meter) might be useful. Or perhaps it would be better to just use mile per Joule (mpJ). Let’s see what this would look like for a internal combustion car that gets 30 mpg. This car would travel 30 miles with 1 gallon of gas (38.9 million Joules). This would be 7.71 x 10^-7 miles per Joule (mpJ). Now we can use this for a electric car. How about a real example like the 2023 Chevy Bolt? This car has a battery that stores 66 kiloWatt-hours of energy (2.376 x 10⁸ Joules) with a range of 259 miles. That’s 1.09 x 10^-6 miles per Joule.
Well, that’s not going to work. No one wants to talk about efficiency using scientific notation. Well, one option would be to convert to miles per kilowatt-hour. With this version, the 30 mpg car would have an efficiency of 2.78 mile per kwh (mpkwh) and the Bolt would be 3.9 mpkwh. This might work.
Another option would be energy per mile — which is just the inverse of the mile per energy. If we use Joules per mile, a 30 mpg car would get 9.47 x 10⁴ Joules per mile (Jpm) and the Bolt would be 9.17 x 10⁵ Jpm. Maybe this doesn’t work either. First, it’s in scientific notation. Second, it’s backwards. You want a car with a LOWER number instead of HIGHER like in miles per gallon.
Range
One of the common metrics right for electric vehicles is to just list the range. I mean, I get it. It’s useful since it let’s you know how far you can go. I mean, who wants a car with a range of only 50 miles when an internal combustion car can easily go 300 or 400 miles. But here’s the problem. Let’s say that you take the Chevy Bolt with a range of 259 miles. That’s great. Now take the Tesla Cybertruck with a listed range of 250–340 miles. These two vehicles have very similar ranges, but are completely NOT similar. The Chevy Bolt obtains its range with a 66 kWh battery where the Cybertruck uses a 123 kWh battery.
It would be like saying your giant gasoline truck has a range of 500 miles but has a gasoline tank that holds 30 gallons. Just because it has a good range doesn’t mean it’s efficient.
Cost Per Mile
We care about the vehicle efficiency because energy isn’t free. When you fill up your car with gas, you probably pay for it (I hope you are paying for that gas). The same is true when charging your electric car — at least someone has to pay for it. It should be easy to measure the efficiency in terms of distance and dollars (that sounds nice). OK, let’s take that 30 mpg car. If you drive 30 miles on one gallon, then you have to pay for that 1 gallon. Maybe the current price of gasoline is 3 dollars per gallon. That means that you are driving 30 miles per 3 dollars or 10 miles per dollar. This is actually crazy to think that it cost a dollar to drive 10 miles, but it’s true. You could also flip this and describe it as 0.3 miles per dollar. It doesn’t help though. What do you do when the price of gasoline goes up to 4 dollars per gallon? Recalculate? No one wants to do that.
You have the same problem with electric vehicles. How much does it cost to charge a Chevy Bolt with a 66 kWh battery? Well, that depends on where you live. In Louisiana, electricity is relatively cheap at 10 cents per kWh. It would only cost 6.6 dollars to charge your car (but it would cost twice that to charge your Cybertruck). However, if you are in Hawaii then electricity is 30 cents per kilowatt hour. It would cost $19.8 to electrically fill it. Honestly, that’s still a pretty good deal. Don’t forget that electric cars could also be charged at home with supplemental solar panels so that it’s possible to get that cost per mile even lower.
Equivalent MPG
Everyone already understands “miles per gallon”. So, what if we just take the energy an EV uses for one mile and convert that to an equivalent amount of gasoline? Let’s try this with the Chevy Bolt. We already know that it has a range of 259 miles and a battery capacity of 66 kWh. Since 1 gallon of gas is equal to 10.795 kWh (3.89 x 10⁷ Joules), then the energy stored in the Bolt is the same as 6.11 gallons of gasoline. This would give the Bolt an efficiency equivalent to 42 mpg. That seems reasonable (surprisingly).
What about the Tesla Cybertruck? With a range of 300 miles (I just picked a number) and a battery storing 123 kWh, it is the same as fuel take holding 11.4 gallons. This would give it an equivalent efficiency of 26.3 mpg. Again, I can believe that. This is actually pretty nice since you can see that the Cybertruck is much more like a normal gasoline truck in terms of efficiency.
For me, it seems like equivalent mpg is the best way to go. Maybe we could even label it as empg (equivalent miles per gallon). Oh, what about electric miles per gallon? Yes, I know they are electric miles but it sounds cool.
