MPGe converts electricity into a gallon of gasoline
The EPA fixes one gallon of gasoline at 33.7 kWh of energy (115,000 BTU). MPGe is how far the car goes on that much electricity. Flip it around for the number people actually pay for: kWh per 100 miles = 3370 ÷ MPGe. A plug-in hybrid quotes both an MPGe (on battery) and a plain MPG (on gasoline), because it runs on either.
MPGe and MPG are not the same yardstick
This is the number-one confusion. A high MPGe does not mean the car is that many times more efficient — it means electricity packs more usable energy per "gallon equivalent." A gasoline engine wastes about 70% of its fuel as heat; an electric motor wastes about 15%. That gap is already baked into the MPGe figure.
The bars compare energy per 100 miles, not dollars. Electricity is cheaper per unit, so the cost gap is even wider than the efficiency gap.
MPGe, energy use and blended running cost
| MPGe (electric) | kWh / 100 mi | Cost / mi at $0.16/kWh |
|---|---|---|
| 80 | 42 kWh | ~$0.07 |
| 100 | 34 kWh | ~$0.05 |
| 124 | 27 kWh | ~$0.04 |
| 140 | 24 kWh | ~$0.04 |
Costs use energy at the wheels; add about 12% for charging losses. Running the same car on gasoline at 40 MPG and $3.10/gal costs about $0.078 a mile — roughly double the electric mode.
What the on-paper number leaves out
- Charging losses. 10–20% of the energy from the wall never reaches the battery. Home Level 2 (about 88%) beats DC fast charging (80–85%) for efficiency.
- Cold weather. Winter cuts electric range 20–40% and cabin heating drains the battery, so more miles fall back to gasoline.
- The blend is what matters. A plug-in hybrid only earns its MPGe on the miles you actually drive electric. Most US drivers stay under 40 miles a day, so a 40+ mile battery can keep you off gasoline most days.
- Real-world gap. Studies find true emissions run 1.5 to 3.5 times the EPA lab figure, mostly because owners drive in the cold and don\'t always charge.
Common questions
How do I calculate MPGe?
Divide the miles you drive on electricity by the kilowatt-hours it took, then multiply by 33.7. For example, 50 miles of range on a 13.6 kWh battery is (50 / 13.6) x 33.7 = 124 MPGe. The 33.7 comes from the EPA treating one gallon of gasoline as 33.7 kWh of energy.
Can I compare MPGe to regular MPG?
Not directly. MPGe rates energy content, not the mechanical efficiency you feel. A car at 120 MPGe is not "three times cheaper to run" than a 40 MPG car, because electricity is far cheaper per unit of energy than gasoline. The real running-cost advantage is closer to 2 to 2.5 times.
How many kWh does a plug-in hybrid use per 100 miles?
Take 3370 divided by the MPGe. At 124 MPGe that is about 27 kWh per 100 miles at the wheels. Add roughly 12% for charging losses and you pay for about 30 kWh per 100 miles from the wall.
