The Volt's 230 M.P.G.: Is M.P.G. Still Relevant?

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Rebecca Cook / REUTERS

GM's Chevrolet Volt, an electric car that gets 230 miles per gallon

Chevy Volt's 230 m.p.g. sounds good, but what about 60 m.p.k. or 25 kWh/100m? The array of advanced car technologies hitting the consumer marketplace has brought enough boasts, confusion and questions to fill a gas-guzzling SUV's cargo hold.

GM CEO Fritz Henderson announced this week that the company's much anticipated Chevy Volt (half electric, half fossil fuel) is the undisputed winner in the miles-per-gallon race, claiming that under new EPA guidelines the Volt will hit 230 miles per gallon (city), the first car to ever earn triple-digit fuel efficiency. Not to be outdone, Nissan fired back a few days later to its Twitter base of fans that its just-announced all-electric Nissan Leaf would be rated at 367 m.p.g., also using EPA guidelines.

By comparison, Toyota's fuel-sipping Prius hybrid looks like an outright gas hog at 51 m.p.g. (city), and the Honda Insight hybrid appears ready for the cash-for-clunkers program at 41 m.p.g. Ditto for the Ford Fusion hybrid (41 m.p.g./city) and Toyota's Camry Hybrid (40 m.p.g./city).

But what about m.p.k. and k.p.m. comparisons? And what about MPGGE and GHGP? It's the new language in your car future.

The EPA's methodology behind the Volt's eye-popping 230-m.p.g. rating, and those of other so-called extended-range electric vehicles (EREVs), is still under wraps — though GM and others claim to be using it — and the agency says it can't comment since it has not yet tested the Volt. In the meantime, the Society of Auto Engineers continues to tinker with its new hybrid test protocols. It has a lot of automotive fans scratching their heads about the recent Volt m.p.g. claims and how pure-electric vehicles and hydrogen-fuel-cell vehicles stack up.

The EPA's National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Mich., says it's revising its formulas to better reflect real-world representations of "driving cycles": that is, up hills, down hills, acceleration rates, city miles and highway miles — the driving conditions that affect fuel efficiency or, in the case of hybrids and electric cars, how long the battery will last. This is why the EPA says it "cannot confirm" GM's mileage claims but is happy the company is innovating such fuel-efficient cars.

With the Volt, GM is among the first to make some marketing hay from the unreleased EPA revisions, which evidently take into account onboard gasoline generators like the Volt's. Specifically, GM bases its 230-m.p.g. boast on a blend of the Volt's electric-only mode — which has a 40-mile-range limit — and charge-sustaining mode, with its 1.4-L electric generator running. (The generator is a small gas-powered engine that keeps the batteries charged while the car is being driven, hence the "extended range.")

Here's the breakdown: The 230-m.p.g. number, according to GM's Frank Weber, global-vehicle-line executive for the Volt, is a measurement of the car's "city-driving cycle" — that's the 40 miles it can go without gas, plus one daily electric recharge, plus a little extra help from the gasoline it might need to continue to charge its batteries when they get low during driving in the city. It's basically measuring the Volt's electric-only-mode (with some help) mileage capacity. If the Volt got out on the highway — where it's powered largely by gasoline — and traveled 200 miles, the m.p.g. would drop like a stone and most likely be more in line with other hybrids.

In the end, the EPA's new EREV testing process, like the one that California's Air Resources Board (ARB) uses, may not actually measure gasoline usage at all but rather kilowatt hours per 100 miles, or kWh/100m. That figure is converted into miles per gallon, which effectively makes miles per gallon irrelevant.

GM says the EPA will weight plug-in electric vehicles as traveling more city miles than highway miles on only electricity, presumably figuring that people buy electric cars primarily for local driving. GM expects the Volt to consume 25 kilowatt hours per 100 miles of city driving. At the U.S. average cost of electricity (approximately 11 cents per kWh), a typical Volt driver would pay about $2.75 for enough electricity to travel 100 miles, or less than 3 cents per mile. (Conversely, a gasoline-powered car that gets 20 m.p.g., for which the driver pays $3 per gallon, has a per-mile fuel cost of 15 cents.)

Also, if you use the electric vehicle's or hybrid's electric-only mode, the miles per gallon are "infinite or have no meaning since no gas is being consumed," according to Anthony Eggert of California's ARB. Of course, it takes oil, coal, nuclear, wind or hydro power to create the energy electric cars are powered on, so even eco saints don't get off free. To complicate matters further, if you want a hydrogen-fuel-cell-powered vehicle, like a Honda FCX Clarity, you will need to measure mileage in m.p.k., or miles per kilograms of hydrogen. The Clarity gets roughly 60 m.p.k.

Eggert is in charge of science and technology policy for the state agency, which sets greenhouse-gas performance standards (GHGP) and develops "miles per gasoline gallon equivalents" (MPGGE) for electric and hydrogen-fuel-cell vehicles so it can rate the amount of harmful CO2 released into the air. It's all very confusing and is likely to get even worse, according to Eggert, who doesn't "even bother" with GM's 230-m.p.g. claim. "We ignore such claims entirely," he says. "We look at total energy consumed so we can figure out how much greenhouse gas is being emitted into the air." That is, the industry may be moving from miles per gallon to miles per bucket of soot or, in ARB's case, grams of CO2 per mile. For example, ARB rates a Ford Explorer at 400 grams of CO2 per mile for greenhouse-gas emissions. A Toyota Prius is rated at 200 grams per mile.

The new carbon-emission ratings are destined to be part of our car-buying futures. Now it's up to ad agencies to make these new performance metrics sound sexy.