Why we need big hybrid SUVs

NEW YORK (CNNMoney.com) -- Outrage. Disbelief. Downright disgust. Those were readers' reactions to our recent story about 13 great fuel efficient cars, which featured several trucks and SUVs.

Many of the emails went something like this: "Did you sell your soul to Detroit? Since when is 16/24 'great' fuel-efficiency?"

2008 GMC Yukon Hybrid

Actually, 16 miles per gallon in city driving and 24 on the highway is really good for a large SUV. That's what the GMC Acadia gets.

Compare that to another big General Motors (Charts, Fortune 500) SUV, the GMC Yukon, which has similar interior space. The Yukon gets 14 miles per gallon in the city and 19 on the highway.

Yes, I know. That's a difference of only 2 miles per gallon in city driving. In overall driving - city and highway combined - the difference between the Acadia and the Yukon is 3 miles per gallon, according to EPA estimates.

Whoop-dee-doo?

Look at it this way, though. Over 15,000 miles - about a year's worth of driving - someone who drives the Acadia will use 148 fewer gallons of gas than someone who drives a Yukon. (By the way, all the EPA estimates used in this article are based on new testing procedures that result in lower, but more realistic, mileage figures for all vehicles.)

Now let's look at another car on the list, the Nissan Altima Hybrid. It gets 34 miles per gallon overall compared to 26 miles per gallon for the similar non-hybrid Altima. That's a difference of 8 miles per gallon.

But someone who decides to buy the Altima Hybrid instead of a regular Altima will save just 136 gallons of gas a year. That's still a lot, but not as big of a difference as a mere 3 mpg improvement in a large SUV.

Vehicles like DaimlerChrysler (Charts)'s Dodge Durango Hybrid and GMC's Yukon Hybrid, both due out later this year, are expected to make an even bigger difference. (We got to read lots of angry email when we wrote about the Durango Hybrid: "You folks are either morons or fools to run an article on a hybrid engine that only gets 25 mpg. What kind of reefer are you clowns smoking?")

I'm not trying to hide from the obvious here. Someone who switches from a non-hybrid GMC Yukon to a Toyota (Charts) Prius will save about 611 gallons of gas a year. That switch would make a huge difference.

But how many people do you think could actually do that?

That assumes that all consumers are able to buy the smallest possible vehicle and that no one who drives a large SUV or other full-sized vehicle actually needs one.

Even as consumers have been ditching mid-sized SUVs for smaller SUVs and cars amid rising gas prices, sales of large SUVs have stayed relatively flat. That indicates that these buyers can't easily switch.

I didn't have to go very far to find a perfect test case: my sister. She lives in Cape Cod, Mass., where it can snow heavily in the winter. She has three boys who all play hockey, a dog and a husband; and she usually has a couple of her kids' friends - and their hockey equipment - tagging along wherever they go. In the summer, they have a boat to tow.

Try fitting that life into a Prius. My sister drives a GMC Yukon XL not because she thinks it's cool but because it does what she needs her vehicle to do even when there's a foot-and-a-half of snow on the ground.

The gas mileage, on the other hand, she's not so crazy about.

But when she goes to trade in that vehicle she might be able to get a Yukon XL Hybrid. GM estimates it should get about 20 miles per gallon overall. That means it would use 237 fewer gallons of gas a year than the Yukon XL she drives now.

That's a lot of gas. And, if that hybrid Yukon XL isn't available, you think she'll buy a Honda (Charts) Civic? No. More likely, she'll buy another Yukon, a non-hybrid one. That would be another 237 gallons of gas per year not saved.

Drawing a big circle around one number - say 35 miles per gallon - and saying "a vehicle isn't fuel efficient unless it gets 35 miles per gallon" is silly, simplistic and pointless. It's counterproductive because it keeps car companies focused on saving fuel where the savings are needed least. The best way to an get eye-popping miles-per-gallon number is by piling advanced hybrid technology into an already-efficient small car. It gets headlines, but it limits the real impact of the technology.

Car companies such as General Motors, which sells 70 percent of all large SUVs, and Chrysler are doing the right thing by putting their best fuel economy technology where it's actually going to do the most good: in vehicles that really need the help.