GM's 230 MPG Estimate For Volt Works, Or Not, Depending on the Drive.

GM's 230 MPG Estimate For Volt Works, Or Not, Depending on the Drive.

By John O'Dell August 11, 2009

We Deconstruct the Claim; Remember - Your Mileage May Vary

By John O'Dell, Senior Editor

I've figured out how to get 230 miles per gallon driving a Chevy Volt around the city.

I've also figured how to go the same distance and get only about 40 mpg, with the same car on the same route.

If that sounds confusing, just wait 'til I try to explain to you how GM came up with its 230 miles per gallon city driving figure for the Chevrolet Volt - apparently with the tacit agreement of the EPA, despite the agency's stated inability to confirm GM's news-making fuel economy claim for its four-place, extended-range hybrid.

First off, though, let's get something straight: That the Volt might or might not get the equivalent of 230 miles per gallon doesn't mean it actually would go 230 miles if you put a single gallon of gas in its tank and sent it out to find its way in the world.

That's preposterous.

Despite what GM calls it, we're talking about a gas mileage equivalency, not real miles per gallon.

Infinity MPG?

Under the rational that GM says the EPA applied to the Volt, an ell-electric car such as the upcoming Nissan Leaf or existing Tesla Roadster would have an official rating of "infinity miles per gallon."

But the EPA doesn't do that. It doesn't rate pure EVs for miles per gallon.

What GM, reveling in the publicity is had received by making the claim, is really saying is that if you drive the Volt in a very prescribed manner you could get the equivalent of 230 miles per gallon out of all of the energy expended to move the car.

The EPA's official position, as we reported earlier today, is that it hasn't done its own testing of the Volt, so can't confirm GM's claim.

But GM has tested, and it has done so using what it says is a "tentative" EPA methodology for EVs and PHEVs.

The Environmental Protection Agency may not be able to confirm it, but we know that GM has the expertise to run a test using the EPA driving cycle and come up with the same figures the agency eventually will release - they do it with just about every vehicle they build, as do most other automakers.

And we have a strong suspicion that GM didn't announce that 230 mpg figure without clearing it with someone high up in the EPA - the company, just out of bankruptcy and effectively owned by the feds, can't afford to alienate the official mileage validator by flying off on its own with preposterous claims.

That Was Then

Hmmmm. Does that last comment mean I think GM's claimed mileage for the Volt isn't preposterous?

In Real Word, it's definitely a claim that's pretty far out there.

But in Car World, where fuel economy claims often are not what they seem - remember, "your mileage my vary" is the official anthem of Car World - 230 mpg is valid, even if hard to swallow.

I asked GM's Volt project manager Tony Posawatz to explain the "tentative" methodology that led to the rather substantial fuel economy claim and he said that GM has an agreement with the EPA not discuss particulars - that the agency wants to wait until it finalizes things and then do the explaining itself.

He also said that while GM is now talking about city mileage, the EPA may wind up issuing only one fuel economy number for vehicles like the Volt - a combined city and highway average.

That, of course, would make moot today's 230 mpg claim for city diving.

GM won't give even a hypothetical combined city-highway number, except to say it will be in the "solid triple digits," meaning, we think, close to 150 mpg-equivalent.

No Accounting for Electricity

Posawatz also said that the EPA method, for some unfathomable reason - modifier and italics mine - doesn't take into account the energy consumed in propelling the Volt with electricity, or in generating the electricity used to charge the batteries.

Only the gasoline consumed during the test cycle is used in compiling the fuel economy figure.

(Remember, we're not talking about an all-electric car, but a plug-in hybrid with a smallish battery that can hold enough juice to propel it 40 miles, at which point, if a drive is longer than 40 miles, a gasoline engine-generator kicks in and produces juice to keep the electric motor running.)

If I understand this correctly, that's why an EV like Nissan's Leaf would get infinite miles per gallon in all driving cycles - because  it uses only electricity; no gas at all.

(The Leaf, by the way, has a Department of Energy rating of around 360 mpg, using a system that figures the gasoline-gallon equivalent of the energy in a kilowatt-hour of electricity; a system the EPA apparently doesn't use.

Deconstructing MPG

So, how does the Volt get 230 mpg?

Drive it 40 miles on Monday -to the point the batteries are depleted and have to be recharged, but not to the point the gas engine-generator comes on.

Then recharge it and drive 20 miles on Tuesday.

Do an all-electric 30 miles on Wednesday, and 30 more on Thursday.

Put 20 miles on the odometer on Friday.

Now you've driven 140 miles and not used a drop of gasoline.

Then on Saturday, take a nice drive in the country, put 80 miles on the odo - 40 electric and 40 with the generator chugging away.

Take it easy on Sunday, maybe a 10-mile round trip to the market, church or Aunt Frannies' for dinner.

And now you've done 230 miles in one week and used approximately one gallon of gas, based on the Volt's 8- to 10-gallon gas tank (they haven't decided on a final tank capacity yet) and GM's claim that the generator will run for 300 miles on a single tank.

So there you have it: 230 miles per gallon.

Or Less

Of course, if you start out Monday morning and put the entire 230 miles on the odometer in one nonstop trip, your fuel economy drops to the equivalent of 38.3 miles per gallon (closer to 40 after factoring in some short periods when recaptured energy in the batteries will allow for brief spurts in which the engine-generator shuts down).

That's because you'll burn through approximately 6 gallons of gasoline to keep the generator turning after you go through that first 40 miles of gas-free driving.

Preposterous?

You tell us.

John O'Dell:  is an AutoObserver Senior Editor. Follow @AutoObserver on Twitter.