MPGe Explained: How Electric Car Efficiency Compares to MPG

Every EV sold in the US carries an EPA window sticker showing a number like “115 MPGe” — a figure designed to look directly comparable to a gas car’s MPG rating. It’s a useful starting point for comparing running costs across fuel types, but it’s an energy-equivalence figure, not a real conversion — and there’s no formula that turns an MPG rating into an EV’s driving range, because the two numbers don’t describe the same thing.

What MPGe actually measures

MPGe stands for “miles per gallon gasoline-equivalent.” The EPA defines a fixed energy equivalence:

1 gallon of gasoline = 33.7 kWh of energy

This number comes from the energy content of gasoline, not from any property of EVs — it’s a unit conversion between two energy measurements (chemical energy in fuel vs. electrical energy in a battery), used purely so EV efficiency can be expressed on the same numeric scale as MPG.

So an EV rated at 115 MPGe travels as far on 33.7 kWh of electricity as a notional gasoline car that gets 115 MPG would travel on one gallon of gas. The “fuel” being measured is electricity, converted into gasoline-equivalent units for the comparison.

Why you can’t convert MPG to EV range

EV range depends on battery capacity (how much energy the car can store, in kWh) and efficiency (how far it travels per kWh) — neither of which has anything to do with a gasoline car’s MPG figure. Two completely unrelated vehicles could have the same MPG rating, but EV range depends on a battery spec that a gas car simply doesn’t have.

If you know an EV’s efficiency and battery capacity, range follows directly:

Range (miles) = battery capacity (kWh) × efficiency (miles per kWh)

For example, an EV with a 75 kWh battery and an efficiency of 4 miles/kWh has a range of:

75 × 4 = 300 miles

You can derive miles-per-kWh from a published MPGe figure by dividing by 33.7:

miles per kWh = MPGe ÷ 33.7

A 115 MPGe car gets 115 ÷ 33.7 ≈ 3.4 miles per kWh — multiply by the battery’s usable capacity to estimate range.

Using MPGe for cost comparisons

Where MPGe is genuinely useful is comparing running costs between a gas car and an EV, since both can be expressed in cost-per-mile once you know local electricity and gasoline prices:

• Gas car: cost per mile = (price per gallon) ÷ MPG
• EV: cost per mile = (price per kWh) × (33.7 ÷ MPGe)

Because electricity is usually cheaper per equivalent-gallon than gasoline, EVs typically come out ahead on this comparison even when their MPGe number looks similar to a gas car’s MPG — but the comparison only works because both figures have been put on the same energy basis (the 33.7 kWh constant), not because MPG and MPGe are interchangeable.

Practical takeaway

• MPGe ≈ MPG in terms of “bigger number = more efficient,” and both use the same 33.7 kWh energy-equivalence constant for EVs — so they’re reasonable for side-by-side efficiency or cost comparisons.
• Neither figure converts to EV range without also knowing the battery’s usable capacity in kWh — range is a separate spec, not derivable from an efficiency rating alone.
• For converting between energy units directly (e.g., kWh to joules for technical work), use the kWh to joules converter; for comparing gasoline efficiency figures across regions, see the MPG vs L/100km guide.