If you're struggling to keep up with rising mortgage payments and record gas prices, here's a way to save hundreds of dollars a month:

Downsize your ride.

You may have to overcome an upside-down loan and the shockingly low resale values of most full-size pickups and sport-utility vehicles -- the prime candidates for such a switch.

But look how quickly the savings add up with a smaller car or SUV:

Lower monthly payments: $250. Let's say you borrowed $30,000 for 60 months at 7% interest. You're paying about $600 a month. Reduce that to $18,000 for 60 months, and you'll pay $350.

Less costly fill-ups: $50. Drive 1,000 miles a month averaging 15 miles per gallon, and you'll spend $120 a month on gas at $1.75 a gallon. Drive the same 1,000 miles averaging 25 m.p.g., and you'll spend $70.

Reduced insurance premiums: $10. The experts we spoke with say you can save 10% to 20%. That means your premiums could drop from $600 to $480 a year for the same coverage.

Total monthly savings: $310.

That's far more than anyone can save by scrimping on things like eating out or going to the movies.

You could cut your costs even more if your current vehicle is out of warranty and you buy a new one or a late-model used car or truck that's still covered by a factory guarantee.

Not having to worry about unexpected repair bills is a big plus, and many 2008 and 2009 cars and trucks are backed with the best warranties we've ever seen.

Of course, there's a trade-off to downsizing your ride -- you'll be getting around in a smaller, less powerful car or truck.

But smaller doesn't mean cramped. And less powerful doesn't mean the best small cars, compact cars, compact wagons and small SUVs aren't a blast to drive.

We think you'll be surprised at how much fun they are -- and how much stuff you can pack into them.

Many people buy a large pickup or SUV for tasks they only do once in a great while, like hauling furniture or tackling a muddy rural road. But they're paying a lot for capabilities they use very little.

Think how much easier it would be to park a smaller SUV at the grocery store or mall. And that's something you do every day.

The biggest hurdle to downsizing your ride will be financial.

Edmunds.com says one of every four auto loans is upside-down, which means the borrower owes more than the car is worth.

Many lenders used to allow you to roll that "negative equity" into a new loan on a new car, lending up to 130% of the new car's value.

But you'll have a hard time finding that kind of financing today.

That makes it difficult to get rid of a costly car or truck without spending thousands of dollars in cash to make up the difference.

The average deficit has grown to $4,300 as the resale value for large pickups and SUVs -- the best candidates for downsizing -- has slumped.

Jack Nerad, executive editorial director and executive market analyst for Kelley Blue Book and kbb.com, the most prominent valuation guide for used vehicles, says full-size SUVs "have declined in value about twice as fast as they normally would in the last 12 months."

CarMax Inc., the nation's largest used-vehicle retail chain, reports that during the company's first quarter, which ran through May, wholesale industry prices (the price wholesalers pay for a vehicle that ends up on a used-car lot) for SUVs and trucks declined nearly 25%. That's almost four times the normal depreciation.

"This is the most rapid depreciation of any vehicle segment that we have experienced in our 15 years," says Tom Folliard, CarMax's president and CEO.

Given that, Consumer Reports recently did a study to determine if, and when, trading in a fuel-thirsty SUV makes sense.

The magazine concluded that it's almost impossible to save enough on gas to make up what you're losing on depreciation if your vehicle is three years old or less.

It recommends holding onto the truck until it's at least five years old, a point when most vehicles are paid off or at least worth more than the remaining balance on the loan.

Edmunds.com has a new Gas Guzzler for Gas Sipper calculator to help you decide whether ditching your truck is a good idea.

But here's the bottom line: If you can't afford the payments, and the insurance, and the gas, then you've got to do whatever it takes to replace your expensive ride, even if you have to take a big hit on depreciation.

You need to cut your expenses now, and this is one way to do it.

By Regan Doherty

Interest.com Associate Editor

and Bill Visnic

Interest.com Contributing Editor