GREEN CAR COMMENT: $/100 miles: Electric era’s new mpg

Richard Truett
Automotive News
November 16, 2009 – 12:01 am ET

Nissan North America says the Nissan Leaf all-electric car will get 367 mpg.No, it won’t.
General Motors Co. says the plug-in hybrid Chevrolet Volt will get 230 mpg.

No, it can’t.

The Leaf’s miles-per-gallon rating will be zero, nada, zilch. The car doesn’t burn a molecule of petroleum. It doesn’t get a miles-per-gallon rating, simply because no gallons are involved. And the Volt’s true fuel economy is more confusing and difficult to calculate because it uses both gasoline and electricity.

“The current system isn’t relevant anymore,” says Matt Mattila, a consultant at the Rocky Mountain Institute, an environmental consulting firm in Denver.

Still, consumers need a simple way to compare vehicles, one that gives them at least some guidance as to which will cost more to drive.
A new calculation

Let’s park the old way of computing miles per gallon for this new generation of electric vehicles and plug-in hybrids. Instead, let’s create a fair, simple and accurate way to calculate the cost of driving 100 miles straight — regardless of fuel.

There are lots of reasons that some folks will oppose our proposed formula. We’ll get to those later. For now, though, here’s how a per-100-mile formula would work.

You start with some assumptions. The first is an average price of a gallon of regular-grade gasoline. Base it on the average price in the United States the previous year. For 2008, I’ve seen various numbers. But one, taken from a Department of Energy Web site, puts it at $3.21.

Next we need an average price for electricity. Across the United States, electricity typically costs between 11 and 14 cents per kilowatt-hour. Let’s go with 12 cents per kilowatt-hour.

Then we determine how much electricity, fuel or both the vehicle needs to travel 100 miles. From our price assumptions, we can calculate how much that will cost. Let’s run some hypotheticals.

If gasoline-powered car X gets 25 mpg combined city/highway, then it needs four gallons to go 100 miles. Voila: It costs 4 times $3.21, or $12.84, to go 100 miles.

If electric vehicle Y can go 100 miles on a single charge and that charge amounts to 44 kwh, then that vehicle’s energy/fuel cost per 100 miles is 44 times $0.12, or $5.28.

By the way, that energy use figure for a highway-valid electric vehicle comes from Henrik Fisker, founder and CEO of Fisker Automotive. He estimates an electric car would need 22 kwh of electricity to go 50 miles. Fisker Automotive’s first car, the $87,000 Karma plug-in hybrid, will be assembled in Finland by Valmet Automotive Inc. starting next year.

Plug-in hybrids such as the Volt admittedly are more complicated to calculate, but not impossible.

GM says the Volt will go 40 miles on electricity before switching to gasoline power, which is directed through the electric motor. So figure the cost for the electricity for 40 miles, then the cost for enough gasoline to drive the next 60 miles. Add the two figures.

A new calculation

Let’s park the old way of computing miles per gallon for this new generation of electric vehicles and plug-in hybrids. Instead, let’s create a fair, simple and accurate way to calculate the cost of driving 100 miles straight — regardless of fuel.

There are lots of reasons that some folks will oppose our proposed formula. We’ll get to those later. For now, though, here’s how a per-100-mile formula would work.

You start with some assumptions. The first is an average price of a gallon of regular-grade gasoline. Base it on the average price in the United States the previous year. For 2008, I’ve seen various numbers. But one, taken from a Department of Energy Web site, puts it at $3.21.

Next we need an average price for electricity. Across the United States, electricity typically costs between 11 and 14 cents per kilowatt-hour. Let’s go with 12 cents per kilowatt-hour.

Then we determine how much electricity, fuel or both the vehicle needs to travel 100 miles. From our price assumptions, we can calculate how much that will cost. Let’s run some hypotheticals.

If gasoline-powered car X gets 25 mpg combined city/highway, then it needs four gallons to go 100 miles. Voila: It costs 4 times $3.21, or $12.84, to go 100 miles.

If electric vehicle Y can go 100 miles on a single charge and that charge amounts to 44 kwh, then that vehicle’s energy/fuel cost per 100 miles is 44 times $0.12, or $5.28.

By the way, that energy use figure for a highway-valid electric vehicle comes from Henrik Fisker, founder and CEO of Fisker Automotive. He estimates an electric car would need 22 kwh of electricity to go 50 miles. Fisker Automotive’s first car, the $87,000 Karma plug-in hybrid, will be assembled in Finland by Valmet Automotive Inc. starting next year.

Plug-in hybrids such as the Volt admittedly are more complicated to calculate, but not impossible.

GM says the Volt will go 40 miles on electricity before switching to gasoline power, which is directed through the electric motor. So figure the cost for the electricity for 40 miles, then the cost for enough gasoline to drive the next 60 miles. Add the two figures.

A new calculation

Let’s park the old way of computing miles per gallon for this new generation of electric vehicles and plug-in hybrids. Instead, let’s create a fair, simple and accurate way to calculate the cost of driving 100 miles straight — regardless of fuel.

There are lots of reasons that some folks will oppose our proposed formula. We’ll get to those later. For now, though, here’s how a per-100-mile formula would work.

You start with some assumptions. The first is an average price of a gallon of regular-grade gasoline. Base it on the average price in the United States the previous year. For 2008, I’ve seen various numbers. But one, taken from a Department of Energy Web site, puts it at $3.21.

Next we need an average price for electricity. Across the United States, electricity typically costs between 11 and 14 cents per kilowatt-hour. Let’s go with 12 cents per kilowatt-hour.

Then we determine how much electricity, fuel or both the vehicle needs to travel 100 miles. From our price assumptions, we can calculate how much that will cost. Let’s run some hypotheticals.

If gasoline-powered car X gets 25 mpg combined city/highway, then it needs four gallons to go 100 miles. Voila: It costs 4 times $3.21, or $12.84, to go 100 miles.

If electric vehicle Y can go 100 miles on a single charge and that charge amounts to 44 kwh, then that vehicle’s energy/fuel cost per 100 miles is 44 times $0.12, or $5.28.

By the way, that energy use figure for a highway-valid electric vehicle comes from Henrik Fisker, founder and CEO of Fisker Automotive. He estimates an electric car would need 22 kwh of electricity to go 50 miles. Fisker Automotive’s first car, the $87,000 Karma plug-in hybrid, will be assembled in Finland by Valmet Automotive Inc. starting next year.

Plug-in hybrids such as the Volt admittedly are more complicated to calculate, but not impossible.

GM says the Volt will go 40 miles on electricity before switching to gasoline power, which is directed through the electric motor. So figure the cost for the electricity for 40 miles, then the cost for enough gasoline to drive the next 60 miles. Add the two figures.

Objections

Sounds simple, right? Here are some of the objections to this method — all valid — and our responses.

1. The numbers will fluctuate wildly. After all, this year’s gasoline prices aren’t the same as last year’s. So the cost-per-100-miles figure will change every year for every vehicle.

True. That will make comparisons across years difficult.

One solution might be to set an arbitrary price for gasoline — say, $3 a gallon — and use that every year in addition to whatever the previous year’s average price was. That would provide a baseline for comparison across all years.

In addition, temperature, geography, traffic patterns and driving styles are going to dramatically affect the distance an electric vehicle can go — much more so than they do for gasoline-powered cars.

2. It’s misleading.

Again, true. But this is an old criticism, not one unique to this method. Every fuel economy test conducted in a lab differs from what different drivers will experience.

Remember how outraged Toyota Prius buyers were when their cars’ mileage fell short of the government number? You can bet some overwrought consumer backed (or led) by a lawyer will cry foul when his vehicle doesn’t deliver the advertised cost per 100 miles.

3. If you can recharge the vehicle using solar power from roof panels, it might not cost anything to replenish the batteries.

If an environmentalist makes this boast, I say: Good for you. How much did your solar panels cost? Using utilities’ electricity rates is the only fair way to get a nationwide cost.

These objections are valid, but aren’t serious enough to shoot down this approach. Remember, we’re not trying to create a measurement that is ironclad, repeatable in test labs or true to the real world.

Today’s miles-per-gallon ratings aren’t real-world-based either. And engineers can disagree on test methodologies until the cows come home. The EPA and National Highway Traffic Safety Administration each has a miles-per-gallon calculation — and they differ.

The goal here is simply an understandable measure that can be used to compare vehicles across power sources. The folks in Washington, working with the industry, probably can improve on this. But from where we sit, dollars per 100 miles — or $/100 miles — seems like a pretty good measurement.

What do you think? Join the discussion at autonews.com/mileage.

You can reach Richard Truett at rtruett@crain.com.