By Dan Strumpf -- AP Auto Writer

NEW YORK (AP) — There's one rule that all car owners learn, usually the hard way: Cheap gas doesn't stay cheap.

As gas prices creep up from their winter lows, that rule is proving itself true again. So if you're counting on your next car to save you money on gas, there are some shopping strategies and new technologies that can help you reach that goal. Here are three tips to keep in mind.

Tip 1: CONSIDER GPM, NOT MPG

Consumers are used to the miles per gallon yardstick for measuring a vehicle's fuel economy. But it's not the only way to think of fuel efficiency. Consider flipping the formula on its head — in other words, "gallons per mile" rather than "miles per gallon" — and you get a much more precise idea of how much gas and money a new car will save you, said professor Richard Larrick, professor at the Fuqua School of Business at Duke University.

A few basic calculations will give you a vehicle's fuel efficiency in gallons per mile. But if you don't care to do the math, Larrick and colleague Jack Soll run GPMCalculator.com, which will crunch the numbers for you.

Here's how it works: First, pick a certain number of miles you drive in a given period of time. Larrick prefers 10,000 miles, because it's nice and round and close to most Americans' annual mileage.

Then, divide that number by a vehicle's mpg. That tells you how many gallons of gas a vehicle burns over that distance. For example, take an SUV that gets a measly 12 mpg. A car owner who drives 10,000 miles per year would consume 833 gallons of gas. In other words, it has a gpm of 833 gallons per 10,000 miles.

Suppose you decide to upgrade to a more fuel-efficient SUV, say one that gets 14 mpg. It doesn't sound like much of an improvement. But by the gpm yardstick, the new SUV clocks in at 714 gallons per 10,000 miles — a savings of 119 gallons of gasoline. At Wednesday's average price for regular unleaded, that's a savings of \$244.

The conclusion: At the low end of the mpg spectrum, seemingly insignificant improvements yield big savings, Larrick said.

But the returns diminish rapidly as mpg climbs. When a car owner upgrades from 28 mpg to 40 mpg, the gas savings comes to 107 gallons per 10,000 miles. From 40 mpg to 60 mpg, it's 83 gallons per 10,000 miles.

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"I really want to get people out of the 15 mpg cars," Larrick said. "Ideally, they end up at the 60 mpg car, but the important thing is the recognition that we should get them from the 15 mpg car to the 30 mpg car."

Tip 2: GET TURBOCHARGED

High-end hybrids and electric cars are garnering the most publicity these days. But automakers are also putting other technologies at work to improve the fuel economy of conventional vehicles.

One example is the turbocharged engine that recycles super-hot exhaust back into the engine to improve the engine's energy efficiency and power.

Until recently, turbocharged engines have largely been confined to European markets and to high-end sports cars, but that is starting to change. Ford Motor Co., for example, has been bringing turbochargers to the mainstream car market with its EcoBoost engine. The automaker plans to introduce the direct-injection turbocharged engine in 90 percent of its models by 2013. Ford says it improves fuel economy by up 20 percent and cuts carbon dioxide emissions by 15 percent without sacrificing power.

"The consensus is that turbocharging will become a mainstream technology in the U.S.," said David Paja, director of marketing for passenger vehicles for Honeywell, which makes the EcoBoost engine.

Turbocharged engines typically carry a premium of \$1,000 to \$1,500 over their conventional counterparts, Paja said, but the extra cost of a hybrid can be thousands more.

A slew of European automakers are rolling out diesel-fuel models in the U.S. with fuel efficiency that approaches that of hybrids. And now that diesel fuel prices in the U.S. have been falling closer to the price of gasoline, diesel vehicles may be best value.

On average, drivers get 20 percent to 40 percent better mileage with diesel fuel, along with 50 percent more power. For example, Volkswagen's diesel-powered Jetta TDI gets 40 mpg in highway driving, according to the EPA.

Mercedes-Benz also sells several "clean diesel" vehicles under its "BlueTEC" badge. Audi and BMW also offer diesel vehicles in the U.S.

To be sure, diesel engines are more expensive to produce, so the cars cost more than their gasoline counterparts. But buyers of many diesel vehicles are eligible for federal tax credits, which are based on a formula that considers vehicle weight, fuel economy and other factors.

Volkswagen's diesel Jetta and Touareg models, for example, are eligible for credits of \$1,300. A Mercedes GL 320 BlueTEC SUV nets as much as an \$1,800 tax credit. More details are available at http://www.fueleconomy.gov.