How to Calculate MPGe

For a hundred years we’ve gotten used to measuring fuel consumption in miles per gallon (MPG) and even as we shift to electric driving that’s a habit that’s going to be hard to break.  

Miles per gallon of gasoline equivalent (MPGe) is an energy efficiency metric that was created by the U.S. Environmental Protection Agency in 2010, as the first plug-in hybrid and electric vehicle models were being introduced to the market. Its purpose is to relate the amount of energy used by alternative-fuel vehicles to that of their traditional gas-powered counterparts.

Why compare electrified driving to gasoline consumption? When MPGe was first introduced, full-electric cars were only just starting to emerge, and “alternative fuel” meant some kind of hybrid – either a mild hybrid or a hybrid you could plug in to charge a larger battery. Relating energy use to fuel consumption made sense.

An Easy Comparison

The EPA also conducted studies with car shoppers that were looking at more efficient, environmentally friendly vehicles and the research showed that participants had a hard time grasping the new math of kilowatt-hours. Shoppers, used to seeing city and highway MPG on vehicle price stickers, found MPGe an easier adaptation.

How is a vehicle’s MPGe determined? The EPA uses a formula to calculate the precise amount of electric energy that’s equal to the energy in one gallon of gasoline. Once that amount of energy is determined, the vehicle’s consumption of energy per distance – gasoline, electric, or a mix of both – can be used to calculate its MPGe. Much like the agency’s gasoline consumption ratings, the EPA uses computer modeling to create a driving cycle for city and highway use.

Plug-In Hybrids or Electric

For plug-in hybrids, MPGe is a mix of the liquid fuel’s tank-to-wheel energy use and how efficiently it uses battery power. For battery-powered cars, the distance travelled per unit of energy directly determines its MPGe rating.

As an energy measure, MPGe doesn’t necessarily provide a good comparison of operating costs. Gasoline is gasoline (well, premium fuel is more expensive than regular fuel, but you get the idea) but the price of electricity can vary widely, as does the percentage of gas and electricity mixed by various plug-in hybrid models.

Since that energy you add to your electric vehicle usually comes from your home's electric meter, you should consider both range and MPGe when comparing electric vehicles. A more efficient car costs less to recharge but you also want to ensure it has sufficient range to get you through your average day – not a problem with most EVs these days.

Why Electric Is So Much More Efficient

The more research you do and the more electric vehicles you compare, the more you’ll realize EVs are significantly more efficient than anything powered by internal combustion. Electric motors are simpler than gasoline engines, with far fewer moving parts and less energy loss at every step. Many electric motors send 80 percent of their energy to the wheels to move the car forward, more than double the efficiency of the majority of combustion engines — which have many more steps between the engine and the wheels.  

It’s just another reason that going electric makes sense.