I have a confession to make. Do you remember the parsimonious Prius I’ve written about, the one that gets 45 mpg?

Scott Burns

Well, it isn’t the only car in the Burns family. Our other vehicle isn’t a Zamboni. And you won’t find it in our garage, because it won’t fit.

It’s a 2002 Chevrolet Suburban.

That’s a big SUV. To be specific, it’s a Z-71 with four-wheel drive and a towing package. This is one tough vehicle. It will take on any rock pile in Big Bend National Park. It will run on Interstate 40 until they shut the road down. Last winter, on a drive from Santa Fe to Dallas, we passed more than a dozen 18-wheelers trashed on the side of the road. We lost count of the passenger cars crumpled against guard railings, overturned and generally battered. But the Z-71 plowed on through. It’s the George Patton of SUVs. It’s also big enough to haul our Airstream, if time ever permits.

Like most Suburban buyers, we didn’t buy it because we love seeing big numbers on a gas pump. We bought it for the same reason we bought all the Jeeps that preceded it — to haul stuff.

It does this at about 13 miles per gallon. So with regular gasoline around $4 a gallon, it costs about 30 cents a mile to keep it moving. The Prius, on the other hand, costs about 9 cents a mile.

The difference is major. Let the gas tank get close to empty, and the Suburban can’t be refilled because most pumps shut down at $75. Small wonder that sales of new SUVs have plummeted and some car dealers are refusing to accept SUVs in trade — they already have plenty, thank you.

And that opens an interesting question. If an SUV is essentially worthless as a trade, is there a gas price that will compel you to simply junk it?

The answer is yes. But not yet.

Here’s the basic math. If your SUV is worthless, it will be worth junking when the cost per mile of feeding it gasoline is greater than the cost per mile for fuel plus the cost per mile of the depreciation on a new vehicle. (The Junking Moment will be higher if you also consider financing costs and insurance, but we’re going to keep it simple here.)

Let’s also assume that a new car will lose about 80 percent of its value in the first 100,000 miles. That means a new $25,000 vehicle will lose $20,000 in 100,000 miles and cost about 20 cents a mile for depreciation. A $30,000 vehicle will cost about 24 cents a mile for depreciation, and a $40,000 vehicle will cost about 32 cents a mile for depreciation. Depreciation is the largest single cost, by far, of owning a car.

To throw the SUV away, we’d need to find a combination of new car depreciation and gas cost per mile that is less than the 30 cents it costs to run the Suburban.

Can it be done?

Not quite.

Subtract the more efficient cost from the less efficient cost and you have the amount you can spend on depreciation. At $4 a gallon, for instance, the difference between my 13-mpg Suburban and a 40-mpg replacement is about 21 cents a mile. So the replacement vehicle would need to cost about $25,000. That isn’t too likely.

If gas goes to $6 a gallon, however, the Suburban would cost 46 cents a mile, 31 cents more than a 40-mpg replacement vehicle. That vehicle could cost nearly $40,000. (A more sophisticated version of this, based on current car values, is available online at www. edmonds.com. Look for its “gas-guzzler” calculator.)

So what’s the answer today?

For the Burns family, it’s simple. Don’t buy a new vehicle. Drive less. Drive the gas-sipper as much as possible, the guzzler as little as possible. Use the SUV for what it’s made for, not for trips to the grocery store.

Questions about personal finance and investments may be sent by e-mail to scott@scottburns.com or by fax to 505-424-0938. Check the Web site: www.scottburns.com. Questions of general interest will be answered in future columns.

—Universal Press Syndicate