Get 100 mpg from a Hummer H3? It depends on how you do the math.

The morning headline in the online version of our local Detroit Free Press blares out “100 mile per gallon truck,” a reference to the IDEA, a concept vehicle developed by Bright Automotive, debuting on paper today, at a press conference in Washington DC.

And as regular readers of TheDetroitBureau.com read, just yesterday, a prototype plug-in hybrid version of the Hummer also claims to be getting 100 mpg.  Hummer boss Jim Taylor told me, “It’s safe to say” that the truck maker, the brand environmentalists love to hate, will need to seriously consider such an alternative energy approach when it’s formally sold off by GM, in the coming weeks. There are no plans to produce this Hummer; it is strictly a concept.

Then there’s the Chevrolet Volt, General Motor’s eagerly-awaited plug-in hybrid – GM prefers the term, extended-range electric vehicle, by the way – which is due to market late next year.  It’s also expected to deliver that Holy Grail number, 100 mpg. The problem here is the President’s Auto task Force has already declared that the vehicle is  too  expensive to be competitive with already on sale hybrids. Still, 100 mpg  is a great headline.

The huge numbers promised for the new crop of plug-in hybrids should be taken with a large shaker of salt.  Mileage numbers aren’t always what they seem.  And in the race to deliver better fuel economy than ever, one has to ask whether the auto industry has discovered the Holy Grail, or is simply delivering a lot of hype by taking advantage of how the numbers are generated, and with the U.S. government looking the other way.

Conceivably, you may never use a drop of gasoline on these new plug-in hybrids.  The idea behind a PHEV is that you can drive up to around 40 miles using the energy stored in its battery pack.  That’s enough, numerous studies show, to satisfy the daily needs of about 75% of American motorists.

But should you need travel a bit further – maybe for a business meeting, or a weekend jaunt to visit family – you’ll have the advantage of being able to keep going, unlike a conventional battery car.  In the case of Volt, its large internal combustion engine will fire up and generate current to keep powering the electric motors turning its wheels.

But will you actually get 100 mpg?  Not a chance.  Once you’re burning gasoline, you’ll get something closer to 40 mpg.  Now, that’s not bad, and actually exceeds the upcoming increase in federal mileage standards.  But it’s still only a fraction of the 100 mpg sticker.  Washington regulators have come up with a complex formula to give credit for those days when you’re just driving on electric power, ignoring  how the electricity is generated in the energy use calculations. Your mileage will vary.

Proponents of electrification would call these calculations fair, skeptics  and people who passed middle school arithmetic, find it more than a bit misleading.

“These vehicles will not truly get 100 miles per gallon of gasoline,” cautions Stephanie Brinley, an analyst with AutoPacific, Inc., “not in the traditional sense.  It’s a great number,” she adds.  “It sounds awesome because 45 mpg just isn’t good enough, anymore, to make you sound like you have a groundbreaking product, anymore.  It’s too close to reality.”

A fair point.  While the industry, as a whole, is frantically trying to figure out how to get the average car, truck and crossover to achieve 35 mpg, there are plenty of new hybrids and diesels that are beating that number – just not enough of them yet.  So, getting a few 100-milers in the fleet has a dual purpose: it makes nice headlines…and it helps improve a maker’s Corporate Average Fuel Economy, or CAFE, numbers.

One might be able to make a fair case for a plug-in, like Volt, or the IDEA concept, many of which will be run on electricity, most of the time.  But “there are plenty of other ways you can get credits” that stretch a vehicle’s fuel economy rating, says Brinley, sometimes well beyond credulity.

Carmakers have gotten credit for such things as “up-shift” lights.  Those little dashboard warning signals that insist you change gears on your manual transmission.  In concept, they make sense because most drivers don’t shift at the optimum point for maximizing fuel economy.  But whether the lights actually ever motivate “greener” behavior is a serious question.

Perhaps the most egregious example of this new math involves the effort to cut our dependence upon import oil by promoting the use of ethanol, in particular E85, a blend of 85% grain alcohol and 15% gasoline.

These days, auto manufacturers are rolling out millions of cars, each year, capable of running on anything from pure gasoline to E85, yet odds are you’ve never used ethanol, other than the light, E5 blend some refiners use to clean up emissions; (it’s 95% gasoline).  In fact, I’d bet no one in your immediate family, your neighborhood or your office has ever consciously pumped a tank full of E85.

So, in an industry where makers fret over every extra penny, why would a company like GM convert products like its big Chevy Suburban to flex-fuel capability, a process that involves such costly things as stainless steel fuel lines and a special fuel tank capable of resisting the corrosive liquid?

Because the 20 mpg V-8-powered SUV suddenly gets 30 mpg credit in the federal CAFE calculations – a 50% bump reflecting the fantasy that half of all owners would run on the alcohol-based fuel. Want to buy some triple AAA rated mortgagebacked securities? Well, here’s the automotive equivalent.

“Because the auto companies are not as stupid as they look, they have chosen to make most of their flexible-fuel vehicles their least-efficient vehicles. So they get the maximum fuel economy benefit,” said Sierra Club lawyer Dan Becker, during a Congressional hearing, in December 2007, on automotive mileage standards.

The even greater irony is that, on average, cars running on E85 will experience a decline in fuel economy of between 20 to 35%, reflecting the lower energy content in alcohol versus gasoline. And ethanol is taxpayer subsidized and is costing you billions of dollars a year.

It’s expected that the push to meet the increased mileage standards Washington has enacted will cost the auto industry tens of billions of dollars.  So expect to see manufacturers seek out every opportunity they can, whether it’s a bonus for using cleaner fuels or brownie points for introducing fuel-saving technologies to make it look like fuel economy is better than it is.

The makers also go out of their way to maximize the results they get when their cars go through mileage certification.

To its credit, the Environmental Protection Agency runs an absolutely rigid testing system.  It relies on specially certified fuels, stored in a precise manner.  Vehicles are run on carefully calibrated dynamometers and repeat the extra same sample driving loops.

But there lies the problem.  The testing procedures are known to every automaker, and these days, the typical computer-controlled powertrain can be tweaked and tuned to deliver the best possible fuel economy under precisely the conditions that exist in the EPA labs. Your milleage may vary.

Nowhere was this more obvious than when the first Japanese hybrid-electric vehicles arrived on U.S. shores, a decade ago.  With conventional vehicles, it was common, in fact, nearly ubiquitous, for consumers to get about 10% lower mileage than was shown on their vehicles’ Munroney, or window, stickers.  But early hybrids often got 20, even 30% less.  The general consensus, among industry watchers was that they had been carefully programmed to match the EPA cycle, even if that meant poorer on-road results.

In 2008, the feds completely redesigned test procedures to make it harder to turn in a great performance for CAFE purposes that couldn’t be matched in the real world.  The biggest adjustments, overall, where made to hybrid-electric vehicle ratings.  Some models saw their sticker ratings drop by 10 mpg or more.

Despite the fact that automakers and automotive regulators feign “shock, shock” at such distortions, don’t expect things to change all that much.  Both sides recognize the trend is towards improved fuel efficiency.  But getting there isn’t always as easy as just passing a new law.  And even if carmakers deliver breakthrough technology, that doesn’t guarantee consumers will race to the showrooms for it.  So, you’ll likely see some creative math when fuel economy numbers are calculated for some time to come.