Think $4 a gallon gas is a shock? Just wait. It may reach $5. What to do? One reaction might be to run out and buy a hybrid. However, some quiet thought and a little financial arithmetic is appropriate before spending your green.

Don’t get me wrong, hybrids use a lot less fuel than their counterparts. Pound for pound, however, they are more expensive than non-hybrid equivalents. Also, questions remain about battery pack life and replacement and the cost of maintenance for a technically complex automobile.

The "king" or, depending on how you view your car’s gender, the "queen" of practical low fuel consumption vehicles is the Toyota Prius with an overall fuel consumption of 45 mpg. Try and get one of these babies for MSRP and you’ll likely be disappointed. Even forking over the dealer "premium" may not deliver a car for a few months. Remember the Mazda Miata and the Honda Civic in the late ’80s and early ’90s? Price premiums of $1,000 and $2,000 were not uncommon.

Honda Civic Hybrid

The Prius has no non-hybrid equivalent. However, on the basis of weight, exterior dimensions and performance, the Toyota Corolla is close.

A fully equipped Corolla will cost $20,000 and a fully equipped Prius will cost, without considering a price premium, $23,770.

The annual fuel cost for these two cars, assuming $4 gas, and 15,000 miles per year, is $2,039 for the Corolla and $1,282 for the Prius, a difference of $757. The difference in price is $3,770. It takes five years and 75,000 miles of driving to deliver a net savings. At $5 gas, the net savings comes at four years and 60,000 miles.

A price comparison of the $22,590, 4-cylinder Toyota Camry SE and the $25,650, 4-cylinder Camry hybrid results in a premium of $3,060 for the hybrid version. The difference in annual fuel cost is $626. It will take almost five years and more than 70,000 miles driving the hybrid to see a net savings. With gas at $5 a gallon it will take 3 years and 52,500 miles to break even. At $5 gas, this hybrid might be smart buy.

Comparing the $20,470 2.5S 4-cylinder Nissan Altima and its 4-cylinder $25,480 hybrid sister, the difference in price is $5,010. The savings in annual fuel cost by buying the hybrid is $537. At $4 gas, this hybrid has to be driven for nine years and 135,000 miles before realizing any net savings. At $5 gas it will take 7 1/2 years and 112,500 miles to break even. This is a no brainer, buy the 4-cylinder non—hybrid and save money.

Comparing the $20,405 4-cylinder Saturn Aura with the $23,900 4-cylinder Saturn Aura hybrid the difference in price is $3,495 and the difference in annual fuel cost is only $177. At $4 gas the hybrid has to be driven for 20 years and 300,000 miles to break even. The choice is clear in this case, buy the non-hybrid.

The Ford Escape offers a 4-cylinder, four-wheel-drive Limited model for $25,945, and the 4-cylinder, four-wheel-drive Hybrid Limited for $32,385, a difference in price of $6,440 and the difference in annual fuel cost is $703. At $4 gas you will have to drive the hybrid for nine years and 135,000 miles to break even. At $5 gas,  your savings in annual fuel cost will be $879 and you will have to drive the Escape hybrid for more than seven years and 105,000 miles to break even. Don’t even think of buying this hybrid on an economic basis.

Honda sells its non-hybrid Civic EX for $20,710 and the Civic Hybrid for $22,600, a premium of $1,890 for the hybrid. Annual fuel cost savings by driving the hybrid is $632. At $4 gas it takes three years and 45,000 miles driving the hybrid to break even, and at $5 gas it takes 2 1/3 years or 35,000 miles. This hybrid is worth looking at.

For those buying a hybrid for reasons other than saving money, this information will inform you to some economics that might otherwise be driven by other considerations, such as concerns for the environment.

If you are planning to buy a hybrid for economic reasons, only one of the above makes sense at $4 gas, the Honda Civic Hybrid. At $5 a gallon, the Toyota Camry hybrid also is a good buy, and the Toyota Prius is a "toss-up."

(The fuel consumption data used here is from the EPA model year 2008 and model year 2009 Fuel Economy Guides. The EPA has improved its testing techniques for 2008 and 2009 and these figures are considered more accurate than previous estimates. The EPA Guide uses 15,000 miles/year and $3.94/gallon of regular gas in their annual fuel cost estimates. It also assumes you drive 55 percent in the city and 45 percent on the highway.

The model comparison prices were derived from a comparison of standard features offered in the non-hybrid and hybrid versions of the same car. Although it was not always possible to make exact comparisons because of differences in the features offered in hybrid and non-hybrid models, they are similar.)

Motoring since 1952, Austin Lynas has owned 59 cars and at one time raced mini-sedans in England. An aerospace engineer, he was instrumental in developing the anti-skid systems used in British military aircraft that preceded today’s ABS systems. Lynas can be reached by e-mail at austin@sandiegometro.com.