With gas prices inching past $4 per gallon, you're probably cursing that thirsty SUV or turbocharged roadster in the driveway. But would purchasing a more fuel-efficient car actually be worth it? We've simplified things and done the math for you -- but the answer is far from simple.

In Pictures: Are Hybrids Worth It?

Because hybrids cost more than gasoline-powered models, it's not necessarily cost effective to buy one, even when taking into account the money you'll save on gas. But, if fuel costs continue to rise, the gas savings will make up the price difference -- right? And what about conventional gasoline-powered models ... is it worth it to go for the smaller, fuel-efficient vehicle instead of the one you have your heart set on? Keep reading to find out.

Do Try This At Home

We're using an easy formula so that you can plug in your own figures. First, take the number of miles you drive in a year (we're using 15,000, which the EPA says is average) and divide it by the car's combined fuel economy (available at www.fueleconomy.gov) to calculate the number of gallons you'll use in a year. Then, multiply that number by the cost per gallon (we're using $3.96, the national average at press time, according to AAA) to calculate how much you'll spend on gas in a year. Do these first two steps for each of the two models you're comparing. To find out how many years it will take for a hybrid to pay for itself, divide the extra money you would spend buying the hybrid by the extra money you would spend on gasoline for the non-hybrid -- et voila!

To Have and to Hybrid

Based on today's gas prices, the Mercury Mariner Hybrid makes the most financial sense if you're deciding between a hybrid and its gasoline-only counterpart. The base model starts at only $1,750 more than the conventional Mariner and should pay for itself in gas savings in just a little over two years. The Saturn VUE Hybrid and Ford Escape Hybrid will each start earning their keep in about five years.

Hybrid sedans have better gas mileage than hybrid SUVs, but the sedans cost a lot more than their gas-only counterparts. For example, the Honda Civic Hybrid's 42.2 mpg average fuel mileage will save you $600 per year, but its base price is almost $7,600 more than the conventional Honda Civic. You would have to drive the Civic Hybrid for at least 12.5 years to start seeing a return on your investment.

Of course, we can't forget the most popular hybrid -- the Toyota Prius. Compared to the Honda Accord, a midsize gasoline-only sedan that costs a bit less, the Prius should take less than a year to make up for the price difference and will save you an additional $1,000 in gas costs.

According to our calculations, the worst hybrid values are the GMC Yukon Hybrid, Chevrolet Tahoe Hybrid and Chevrolet Malibu Hybrid, which will each take at least 16 years to earn back the extra dollars they cost. In fact, the Yukon and Tahoe Hybrids cost nearly $15,000 more than the conventional models. The Malibu Hybrid will only save about $168 per year on gas, an increase that won't easily make up for the hybrid's nearly $2,800 price jump.

Conventional Costs

A lot of consumers are looking at fuel-efficient gas-only cars to save on gas. While that's a good strategy, it pays to be realistic about how much you'll actually save. Sporty drivers may be thinking twice about buying that costly BMW 3-Series and considering the 32 mpg MINI Cooper instead. The MINI will save you 10 miles per gallon, which translates to about $786 per year or $15.13 per week. Just make sure that $15 a week will make up for the MINI's much smaller interior and cargo area.

You may also be considering dropping a large SUV for a midsize model. Opting for the Honda Pilot instead of the Chevrolet Tahoe will save over $478 in fuel per year. Still, the annual gas savings comes out to just $9.20 per week, which may not be enough to justify trading down. Likewise, the all-new Smart Fortwo's excellent 36 mpg average fuel economy saves $311 per year in gas compared to the Honda Fit. However, the yearly gas savings only translates to about $5.99 per week, which may not be worth the trade-off -- especially when taking into account that the Smart requires premium fuel.

Even when you think two cars are similar, it can still pay -- literally -- to take a closer look. Choosing the Honda Civic over the Honda Accord will still get you seating for five and plenty of convenience features. But the Civic will only save $273 per year on gas -- $5.25 per week -- so ask yourself if it's worth losing the Accord's sportier performance and nicer interior.

The Big Question

So, is going green worth it? The answer, as you may have learned by now, is that it all depends on what car you're buying. The key is to choose wisely, consider your options carefully, and always take time to do the math.

Read More About Fuel-Efficient Cars:

- Hybrid SUV Roundup

- Fuel-Efficient three-row SUVs

- Best Used Fuel Efficient Cars from Consumer Reports

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