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Understanding MPGe: The magic number 33.7, and how to compare fuel efficiency properly

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MPGe: miles per 33.7 kWh of battery

If you've owned a car in the last 20 years, you're probably familiar with the term MPG, or miles per gallon that your car will drive. Fuel efficiency has been a consideration when buying a car for a long time, but ever since gas prices started to skyrocket and the Prius showed the world it was possible to get 50 or more miles per gallon, more and more people are prioritizing efficiency for their next car.

If you're considering a plug-in hybrid or all-electric car, then you may have seen a new acronym that looks almost identical: MPGe. You'll see this directly on the "Monroney label" of the car. Maybe you thought intuitively that the "e" stands for "electric". Maybe you've also noticed that the number tends to be a lot higher than MPG numbers you're used to seeing. Some are even over 100. What is this funny MPGe thing? It actually stands for "miles per gallon equivalent," and it actually means miles per 33.7 kWh battery. A car with 90 MPGe can travel 90 miles with 33.7 kWh of battery charge.

The "e" stands for equivalent, but MPGe is not comparable to MPG at all

With gas cars, it makes sense that a little car gets better fuel efficiency than a giant heavy truck. The same is true with electric. If you put the same size battery in a little electric car, it's going to be able to travel a lot farther than a truck like the Ford F-150 Lightning. It would be nice to be able to quantify this difference so that electric car shoppers can compare between models. MPGe was born.

MPGe was a little too cute for its own good, though, trying to pick a number that was "equivalent" to its MPG counterpart. The idea behind "equivalent" is that the energy capacity of 1 gallon of gas is equal to 33.7 kilowatt-hours of battery capacity. We forgive you if your eyes are starting to glaze over. But it makes a lot of sense that if you're comparing a gas car to an electric car, it would be nice to have a number that's comparable between the two.

Unfortunately MPG and MPGe are really not comparable at all in practice. The 33.7 number is based on the theoretical energy capacity of gasoline, not the practical amount once you account for the substantial energy loss in converting that gas into power for the car. As a result, MPG and MPGe are totally different scales. The MPGe of the worst EV is better than the MPG of the best gas (non-hybrid) car.

A few real-world examples: Toyota RAV4, Hyundai Santa Fe, Chrysler Pacifica, and Ford Escape

The easiest way to see the problem with MPGe meaning "equivalent" is when you're looking at a car that comes with different engine options. Let's take the Toyota RAV4 as an example, since it is one of the most popular vehicles on the road. The RAV4 comes as a regular gas car, a hybrid, and a plug-in hybrid, so we see the full range of options.

If MPG and MPGe were comparable, you would expect the numbers across the different engine options for the same vehicle to be roughly the same. But they're not at all. The RAV4 Prime (plug-in hybrid) is rated at 94 MPGe but only 38 MPG, a difference of 2.5X. This is due to the fact that the gas engine loses lots of its potential energy as it runs. The numbers are even starker if you compare the 94 MPGe to the 29 MPG that the all-gas (non-hybrid) RAV4 gets, a difference of 3.2X. So comparing MPGe vs. MPG is not very helpful. They're not even comparable for different versions of the exact same car! 🤦

Plug-in Hybrid vs. Hybrid vs. Gas Efficiency
Vehicle Plug-in Hybrid MPGe Hybrid MPG Gas MPG
Toyota RAV4 94 38 (2.5x) 29 (3.2x)
Hyundai Santa Fe 76 33 (2.3x) 26 (2.9x)
Chrysler Pacifica 82 30 (2.7x) 20 (4.1x)
Ford Escape 101 40 (2.5x) 30 (3.4x)

Compare like-to-like and you'll see the real differences

The lesson here is that you really can't compare MPGe and MPG numbers. If you're shopping for an electric or plug-in hybrid vehicle, compare the MPGe number from one car to the MPGe number of another. These are truly comparable numbers. If you're shopping a hybrid vs. a gas, then compare MPG. You can't mix and match.

There are actually significant differences in fuel efficiency for all-electric vehicles. Here are a few of the best and a few of the worst. They probably won't surprise you too much.

Most and least electricity-efficient electric vehicles
Vehicle MPGe (miles per 33.7 kWh battery)
Hyundai Ioniq 6 (Long Range RWD) 140
Lucid Air Touring (AWD) 140
Tesla Model 3 (RWD) 132
Tesla Model Y (AWD) 122
Ford F-150 Lightning (Platinum) 66
Rivian R1T (Quad Motor AT Tires) 64
Audi SQ8 e-Tron 63
GMC Hummer EV (MT Tires) 50

Note how big the gap is between best and worst. With the same battery capacity you can go almost 3 times farther with the Hyundai Ioniq 6 compared to the Hummer EV! Most of this comes down to the weight of the vehicle, with some contribution from the tires and vehicle shape as well.

With EVs it's also common to see different wheel and tire options that offer different efficiency. The new sport of optimizing EV range has sparked new innovations in aerodynamics, and it turns out that wheels and tires can make a 10-15% difference.

Range depends on efficiency and capacity

Range is one of the top considerations for electric car buyers, so we need to mention the obvious after all of this discussion about efficiency: range depends on the size of the battery too. Just like how automakers put bigger gas tanks in less efficient gas cars, they also put bigger batteries in less efficient electric cars.

But efficiency does determine cost

However, just like with gas cars, a more efficient car costs less to operate. At the time of this writing, the average US price for electricity is $0.15 per kWh. The average price of regular gas is $3.27 per gallon. Let's revisit that same table above, but this time looking at cost of fuel per 10,000 miles driven (roughly average amount driven per year).

Cost per 10,000 miles driven (Electricity vs. Hybrid vs. Gas)
Vehicle Electric $ / 10,000 mi Hybrid $ / 10,000 mi Gas $ / 10,000 mi
Toyota RAV4 $538 (94 MPGe) $861 (38 MPG) $1,128 (29 MPG)
Hyundai Santa Fe $665 (76 MPGe) $991 (33 MPG) $1,258 (26 MPG)
Chrysler Pacifica $616 (82 MPGe) $1,090 (30 MPG) $1,635 (20 MPG)
Ford Escape $500 (101 MPGe) $818 (40 MPG) $1,090 (30 MPG)

Electricity and gas prices vary substantially in different parts of the US and world, but at least on average it is way cheaper to drive on electricity than gas.

One last thing on cost: if you expand the "Tax credits & savings" section at the top of the All Vehicles page on EV Table, you'll see an option to factor in gas savings to the price. You might choose to shop this way because if you buy an all-electric or plug-in hybrid vehicle, you'll save money over time in operation. With the example of the RAV4 above, it is easy to see the gas savings because there's both a gas and plug-in version. But what about all of the other all-electric cars? There's no gas version of the Tesla Model 3, and as we learned, MPGe and MPG are not comparable. So how do we know how much money you're saving on gas if we don't know how much gas it would take to operate the Tesla Model 3 in the first place?

There's no one right answer. For simplicity's sake we simply divide the MPGe number by 3.5 to get a "comparable" MPG figure, and then calculate the potential savings based on those numbers. This works pretty well on average, but there are exceptions where it may under- or over-estimate gas savings.

Sometimes the best way to get a feel for the numbers is to play with them yourself. Head over to the EV Savings Calculator and you'll be able to customize the calculation based on the vehicles and pricing you're considering.

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