Wh/Mile is Better Than Miles/kWh or MPGe, Because Distance Per Energy Unit Deceives Our Brains

Until very recently, I thought that it didn’t matter what units you used to express the fuel economy or electric efficiency of a vehicle. We have all sorts of measurements, like miles per kilowatt-hour (miles/kWh), watt-hours per mile (Wh/mi), and even MPGe. When comparing the efficiency of one vehicle over another, sometimes their efficiency stats are expressed in different units, and I thought just converting them to my preferred unit for comparison was fair.

For the who-knows-how-many-eth time, Jason at Engineering Explained showed me the error of my ways. It turns out that like any statistic, the numbers can be deceptive. Like the confusion most of us experience when we hear that a man drowned in a river with an average depth of only six inches, our brains can misinterpret what a number really means.

I’m not going to recap Jason’s whole video. I mean I could to get my word count up, but then I’m not doing you any favors. It’s really worth the watch so you can see how the numbers deceive.

He starts out talking about miles per gallon to explain everything, and then ties it all together with electric vehicle measurements in the end, so if you’re an “EVs are the best, and nothing else matters” person, don’t bail out on the video the moment he talks about gallons of gasoline.

The key difference (and the substance of what we’re talking about here) between MPG and gallons/100 miles is the same as the difference between miles/kWh and Wh/mile. One unit measures distance per unit of energy (gallons for gas, kWh for batteries) and the other measures energy used (gallons or Wh) per distance traveled. It’s the expression of the efficiency number that puts our brains in the wrong mode for meaningful comparisons, regardless of the units involved.

For the readers outside of the US who use the SI system, the same is true even for kilometers, because like a mile, a kilometer is a measure of distance. km/kWh is still telling us distance per unit of energy consumed, and Wh/km (like Teslas outside of the US display on screen) is still energy consumed per distance. The same principle still applies, even if Americans use silly measurements*.

One minor difference in gas car measurements vs common electric car measurements is how the numbers are simplified. To keep things simple, it’s common for the energy per distance measurement to be exaggerated to prevent the use of decimals or fractional energy units. For example, it’s common to use liters per 100 km, and not liters per km, because liters/km would often yield a number smaller than 1. For EVs, Wh/mile is easier to use than kWh/mile, which again would end up being smaller than 1 in many cases.

The engineers who decided what units to use were smart enough to help us out by using an easier unit.

Why Distance Per Unit Deceives Us

The deception happens when we go to compare increases or decreases in these statistics. Increasing the efficiency of a vehicle by 1 MPG or 1 mile/kWh doesn’t always mean the same thing.

Jason gives the example of adding 1 MPG of efficiency to a vehicle, and that 1 MPG saves a lot more fuel from 5-6 MPG than it saves from 50-51 MPG. Why? Because a vehicle getting 5 MPG uses 3.33 gallons of gas more than a vehicle that gets 6 MPG, while going from 50-51 MPG only saves .04 gallons. It’s 1 MPG on both cases, but the amount of fuel saved (and thus emissions avoided) differs wildly, and we don’t see which fuel savings are really worth pursuing.

The same happens with EVs. The difference between 1 mile/kWh and 2 miles/kWh is drastic. That’s 1000 Wh/mile vs 500 Wh/mile. The difference between 4 miles/kWh and 5 miles/kWh is only a reduction from 250 Wh/mile to 200 Wh/mile. Thus, a mile/kWh up or down can mean very different things while Wh/mile tells us the whole story of energy consumption as we make comparisons.

For that reason, it’s better for us to use energy per distance measurements. It’s easier to be honest with ourselves.

Seeing Which Efficiency Gains Are Actually The Most Environmentally Valuable

By switching away from MPG, we can be smarter about which efficiency gains are going to have the greatest environmental impact.

For example, let’s look at a semi-truck and a small diesel car (to be consistent about fuel for a truly fair comparison). If a semi-truck getting 8 MPG gains 2 MPG of efficiency through side skirts and an aerodynamic tail, that’s a big gain. At 8 MPG, the truck uses 12.5 gallons of fuel every 100 miles. At 10 MPG, it uses only 10 gallons for the 100 miles. That means it saves 2 gallons/100 miles.

If a little diesel car gets 50 MPG (#Dieselgate completely aside, let’s not get distracted here), that means it’s only using 2 gallons of fuel for the 100 miles. To save as much fuel (and therefore emissions, assuming the equipment isn’t cheating), the little car would have to stay still and go nowhere, or get pulled by an EV across the 100 miles. Even going from 50 MPG (like a diesel Jetta, pre-recall) to 100 MPG would only save a gallon of fuel.

When we think in terms of MPG, MPGe, or miles/kWh, we don’t see which upgrade would save the environment more. We’re far better off to improve the truck only 2 MPG than we are to double the MPG of the little diesel car.

Now, let’s talk electrification. Which saves the environment more, converting the semi-truck to electric, or switching the diesel Jetta out for a Tesla Model 3? If we use MPGe, we’d think we’re saving big time by going from 50 MPG to 134 MPGe. After all, we’ve gone up 84 MPGe! That’s huge, right? Not so much.

Taking the Jetta off the road only saves 2 gallons of fuel per 100 miles from being burnt and sent into our lungs. Taking the semi-truck of the road and replacing it with a Tesla Semi gets rid of 12.5 gallons of diesel per 100 miles driven. Heavy-duty trucks represent only 4.3% of the vehicles on the road, but generate 23% of the transportation-related pollution in the United States, so their impact is far greater.

Using energy/distance instead of distance/energy lets us see that right away. The Tesla Semi and the long-range electric trucks from other manufacturers clearly can’t come soon enough when we consider all this.

Featured image: Jason crunches all of the numbers so we don’t have to. Screenshot from his video (embedded above).

*Side note: Americans actually only use imperial measurements for very few things. Almost all of my wrenches are millimeters these days. Most industrial and scientific work is done with SI units. Only consumer-facing things like milk jugs or meat are sold by the pound, and roads use miles (with the exception of Interstate 19 in Arizona, which uses kilometers for most signage). Some roads near the border with Mexico have dual signage for speed limits, with both kph and MPH on signs. Kids frequently mix imperial and SI units in the classroom, which shows that many of us are bilingual in our measurements.

We are still rednecks, though. I won’t dispute that.