kWh/100mi vs miles/kWh vs MPGe: How to Read EV Efficiency

By Gregory Wilson · Excerpt from Chapter 4 of EV Curious?

With gas cars, we mostly understood miles per gallon. Twelve MPG is terrible, fifty MPG is excellent, done. EVs throw three different efficiency units at you, and different cars display them differently. Here's how they actually relate.

The three units, untangled

The most useful unit for an EV is miles per kWh (mi/kWh). It's the direct equivalent of MPG. A really efficient EV might get 4+ miles per kWh; a less efficient one might only manage 2.5. Just like with gas cars, bigger and heavier vehicles with more power use more energy.

Kilowatt-hours per 100 miles (kWh/100mi) is the same number expressed differently. Many EVs default to this display because it scales nicely (smaller numbers = better) and matches how energy companies bill you. If your car gets 4 miles per kWh, that's 25 kWh/100mi. If it gets 2.5 mi/kWh, that's 40 kWh/100mi.

MPGe (Miles Per Gallon Equivalent) is the EPA's attempt to let you compare EVs to gas cars in familiar terms. It converts the energy in one kWh to the equivalent energy in a gallon of gasoline. Most modern EVs land at 100–130 MPGe, which sounds impressive next to a gas car's 25–35 MPG. But don't get too hung up on MPGe — it's just a unit conversion, and miles per kWh is more useful for understanding actual energy use and charging costs.

The basic range math

Once you know your car's miles-per-kWh number, range is just multiplication. Get 3 miles per kWh and have a 75 kWh battery? Expect roughly 225 miles of range under normal conditions (3 × 75). The complication is that EV efficiency varies more dramatically with conditions than gas car efficiency does.

Cold weather: the range killer

Temperature has a huge impact. In freezing weather, your range can drop 20–40% compared to mild weather. Two things are happening: the battery itself becomes less efficient at low temperatures, and the cabin heating system draws significant power (gas cars get "free" heat from waste engine heat, EVs do not).

Most modern EVs let you pre-condition the battery and cabin while still plugged in. This warms the car using grid power instead of depleting your battery. Some EVs will start preconditioning automatically if you set a departure time. It's like remote-starting a gas car, but much more effective. Heat affects efficiency too, but usually not as dramatically as cold.

Speed and aerodynamics: the math gets brutal

EVs are designed with extremely low drag coefficients. The Mercedes EQS hits 0.20, the most aerodynamic series production car ever made. That focus pays off — but it also means driving speed has an outsized impact on efficiency.

Drive a Chevy Bolt at 55 mph and you might see 4 miles per kWh. Take that same car up to 80 mph and you might only get 2.5. Aerodynamic drag scales with the square of speed, and the power needed to push through it scales with the cube. Doubling speed from 40 to 80 mph creates four times the drag and requires eight times the power. So going from 70 mph to 75 mph isn't a small change — it can increase energy use by around 15%.

Headwinds matter for the same reason. I learned this on a trip from San Francisco to Seattle when my battery dropped faster than expected. Strong northern winds were blowing directly against my route. Headwinds affect gas cars too, but you notice them more in an EV because the energy display is right in front of you.

Terrain, towing, and driving style

Mountains use more energy on the way up; regen recovers some on the way down. City stop-and-go is actually quite efficient in an EV thanks to regenerative braking — the opposite of gas cars where highway driving is most efficient.

Towing is brutal. A trailer can cut your range by 50% or more, depending on its size and shape. A boxy travel trailer is especially bad because it dramatically increases drag at highway speeds. Most EVs don't know they're towing, so the range estimate won't reflect it — watch the actual mi/kWh reading and plan charging stops conservatively.

Driving style matters a lot. Accelerate gently and you'll see great efficiency numbers. Drive like you're auditioning for a Fast & Furious movie and your range plummets. The instant torque of EVs makes it tempting to use all that performance, but it comes at a real cost.

EPA range vs WLTP vs the real world

The published range numbers can be confusing. In the US, the EPA test is done in controlled lab conditions that don't quite reflect real-world driving. Europe's WLTP cycle typically produces higher range numbers — my Volvo EX90 has an EPA range around 300 miles but a WLTP rating around 360. In my experience, EPA is much closer to what I actually see on the road; WLTP overstates real-world range by 15–25% in most cases.

Publications like Edmunds, Out of Spec Reviews, Car and Driver, and MotorTrend now publish a standardized 70 mph highway range test alongside the EPA number. The methodology is simple: charge to 100%, run at a steady 70 mph in mild weather until empty, measure how far it went. The EPA cycle is heavily weighted toward city driving, so it tends to undercount city range and overstate long-interstate range. The 70 mph test isn't perfect (it ignores wind, weather, and elevation), but if you're comparing two cars for road-trip duty, that number is usually a lot closer to what you'll experience.

Why your range estimate doesn't match the math

If your EV has a 300-mile EPA range and you're sitting at 50% battery, you might expect to see about 150 miles of estimated range remaining. Instead, you might see 118. Your battery isn't broken — your car is being smart.

Unlike a gas gauge that just shows fuel remaining, your EV's range estimator constantly recalculates based on your recent driving. If you've been driving aggressively or tackling hilly terrain, the car learns and adjusts. It's essentially saying, "Based on how you've been driving lately, here's how far this remaining charge will actually take you." This is more useful than simple math would be — you want a realistic estimate, not an optimistic one.

The estimate will also change based on your planned route if you've entered a destination. About to climb a mountain? The estimate drops. Heading downhill? It rises. Don't panic at the fluctuations — your car is trying to give you the most realistic information it can, rather than false confidence based on lab testing.