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Like other full-size GM trucks, the Yukon is a comfortable, capable hauler and puller, but in a world of ever-rising gas prices, gas mileage has become a real concern for all trucks.

Into that climate comes GMC's new Yukon Hybrid, a 4-wheel-drive truck that promises 20 mpg in town and on the highway. Compare that to a standard Yukon Denali 4WD that is rated 13 mpg city and 19 highway, and you'll see that the Hybrid is substantially more efficient.

The big gain here is in city driving.

Rocky Mountain test ride
We put GMC's gas mileage figure and the Yukon Hybrid's performance to the test last spring in Colorado. GMC loaned American Snowmobiler one for a week as we tested sleds for this season, and we weren't shy about loading up the truck.

Four of us and our snow gear piled into the Yukon Hybrid at the Denver airport and headed through the Rockies to Grand Lake, a resort town at about 8,500 ft. of altitude. Through the week we ran to and from our test site, rode around town and never had fewer than four people, plus gear, in the truck.

First, on the mileage front we averaged 18.1 mpg, according to the Yukon's trip computer, but my figures showed us closer to 19.4 mpg, excellent for fully-loaded big truck climbing mountain roads. Our regular GMC Sierra All Terrain got 17.1 mpg in our best highway drive in the flat Midwest with no trailer and 13 mpg with a trailer in tow.

The numbers become more amazing when you consider the Yukon made several trips over steep mountains, pretty stressful on the engine and gas mileage normally. In these conditions, and at high altitudes, usually gas disappears quickly.

How's this good mileage happen? Well, GM gives this and its cousin, the Chevrolet Tahoe Hybrid, a two-mode hybrid system that is really more like three. First, you start off in a continuously variable transmission mode with electric power coming from a 300-volt nickel-metal-hydride battery system. It helps you get rolling smoothly and quietly and saves you the most gas. Getting a 5,800 lb. truck moving is where you use the most energy.

Another mode uses the CV system and another planetary gear system while coasting or braking, which regenerates electric power. After that, the transmission selects when the electric motors will work and when the standard high-efficiency 6.0-liter V-8 needs to kick in. The battery puts the power to those electric motors and the gas engine has Active Fuel Management, which means it can run on 4 cylinders instead of 8 when the truck is cruising.

All this adds up to gas savings.

How does it perform?
You have to get used to the quiet starts. You turn on the truck and only electric power kicks in, so you may crank it a time or two thinking nothing is running. After a couple times trying to start an already running vehicle you'll figure it out. Also the gas engine cuts out at low speeds or when the truck stops, so it's quiet again.

Acceleration is fine, but not meteoric, from a stop. If you need a lot of power and press the accelerator hard enough, the gas engine kicks in more quickly after the electric starts you off, naturally, that'll hurt your gas mileage. Once you're up to 20-30 mph you'll notice no difference in performance from a standard Yukon.

Handling, ride and braking are fine, as in a gas-only Yukon. Plus, the interior is comfortable for five adults. We can't say anything about the hybrid's towing ability as we didn't have a chance to test that. Maybe this winter! But GM says it'll tow 6,000 lbs., which would meet many snowmobilers' needs. We normally see a drop of about 5 mpg when towing trailers with our standard GMCs with the 6-liter V-8. Not sure how towing will affect the Hybrid.

Pricing? The 2008 Yukon Hybrid starts at $52,855 compared with $49,480 for the gas-only Yukon Denali 4WD. That's not a major premium, but you won't make back your money quickly unless you drive a lot of city miles. You gain little on the highway end. For folks who don't care what brand is on the grille, Chevy's Tahoe Hybrid starts about $500 less. Note too that for 2009 the Yukon Denali Hybrid's engine will be a 6.2-liter V-8 that creates 403 horses.
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Weight adjustment
The good news is a hybrid system helps any vehicle get better gas mileage. The bad news, they all weigh more than a standard gas-powered engine alone.

For instance, the GM hybrid weighs about 350 lbs. more, so to help make it more competitive, and to help improve gas mileage even more, GM shed some big pounds with Yukon and Tahoe. In fact, GM cut about 350 lbs., to make it a wash weight-wise with the gas-powered unit.

How? First, GMC uses aluminum in the hood, liftgate, front bumper beam, shaft and engine. In addition, the battery is smaller, there's no starter motor and the wheels and seats are all lightened.

Likewise, the body got some tweaks so it'll slide through the air cleaner. Changes include a rear spoiler, restyled front and rear fascia and wheel spats, a deeper front air dam, running boards and D-pillar moldings.

Every little bit helps! Now if GM will use some of these on the gas- powered models, they too might be a tad more fuel efficient.

The payoff
How quickly will this hybrid pay for itself?

Our calculations show a $3,375 price difference between the gas and hybrid models. So figuring the national driving average of 12,000 miles a year for the truck, and getting 20 mpg while using $4 per/gal. for gas, you'd save $600 a year over the gas version, so it would take 5.6 years to break even.

If, for example, you pay $4.50 a gallon under the same circumstances, you'd begin saving money after 4.2 years.

Additionally, the more miles you drive and/or the more you pay per gallon of gas, the quicker you reach the breakeven point.