There are currently only a handful of gas-electric hybrid SUVs, but more will follow in the coming years. The Lexus RX 400h was the first luxury hybrid, while the Ford Escape Hybrid became the first domestic hybrid SUV. Both models now have twins on the market: the Toyota Highlander Hybrid and the Mercury Mariner Hybrid.

As you drive the Lexus RX 400h, a screen in the instrument panel illustrates how its complex gas-electric hybrid system is working. + enlarge image  |  view gallery >

Just like with hybrid cars, hybrid SUVs have battery-powered electric motors that supplement a regular gasoline engine to economize fuel and reduce exhaust emissions. The Lexus RX 400h and Toyota Highlander Hybrid employ three electric motors to optimize engine power while not compromising utility, such as hauling and towing capabilities. Also like hybrid cars, hybrid SUVs often employ a continuously variable transmission, which doesn’t have traditional gears for different engine speeds, but uses a chain or belt that varies in diameter.

None of the hybrid SUVs currently available were designed for serious off-road duty, so if you plan on tackling tough terrain, another SUV might serve you better. Hybrid SUVs shine in city driving, where the electric motor can work on its own, without the gasoline engine in stop-and-go traffic, even shutting down when the vehicle comes to a temporary stop at a traffic signal or in gridlock. The gas engine works hardest whenever extra power is needed, such as with towing, driving uphill, passing other vehicles and driving fast for sustained periods.

This is why hybrid SUVs tend to be more fuel-efficient in city driving than on the highway, as the chart above illustrates. Because SUVs are heavier and less aerodynamic than cars, there's less potential to improve fuel economy at higher speeds. And as with any SUV, opting for four-wheel-drive reduces fuel economy due to the added weight of the mechanicals and the extra power needed to run all four wheels.

This is a picture of the Ford Escape Hybrid's electric power train, including regenerative braking components. + enlarge image  |  view gallery >

One of the most ingenious aspects of hybrid vehicles is the way the gas and electric motors work symbiotically. While the electric motor kicks in to save fuel, energy from the gas engine that is dissipated while coasting is captured and used to recharge the electric motor's batteries. Similarly, energy dissipated during braking is captured and stored in the batteries. This is called regenerative braking.

There has been much publicity over consumers being disappointed with the actual fuel economy they get with hybrid vehicles. But the truth remains that hybrid cars and SUVs are indeed more efficient than non-hybrid vehicles. The discrepancy in fuel economy figures arises from the fact that the numbers supplied by the EPA and the manufacturers are estimates. And bear in mind that driving style significantly affects fuel economy. Drivers with lead foots burn through more gas than motorists who accelerate moderately and drive at moderate speeds.

The main drawback to hybrid vehicles is that they cost significantly more than their non-hybrid counterparts (note the examples above). Those who buy one in 2005, can declare a $2,000 deduction on federal income tax returns. The clean-fuel vehicle tax deduction will be reduced to $500 in 2006 and will end in 2007.

Tax deductions notwithstanding, at current gas prices it could take a decade or more to recoup the premium you'll pay for a hybrid SUV, depending on how much you drive. What helps to justify the cost is the notion that in owning a hybrid vehicle, you are reducing humanity's dependence on a non-renewable resource, the mass consumption of which is damaging the environment and lowering our standard of living (through toxic pollution and global warming).

So far, hybrid models have proven just as reliable as equivalent non-hybrid models. Though there is debate regarding maintenance costs of the complex hybrid-electric systems, as well as replacing the batteries over the long-run, there isn't enough statistical data yet to determine how these should factor in to your purchase decision.