This is a guest post from Robert Brokamp of The Motley Fool. Robert is a Certified Financial Planner and the advisor for The Motley Fool’s Rule Your Retirement service. He contributes one new article to Get Rich Slowly every two weeks.

Over the holidays, we said good-bye to an old family member. It was definitely her time to go. She leaked, she conked out at odd times, and she stank. Of course, I’m talking about our old vehicle: a minivan with 182,000 miles on it. I didn’t let go of her easily; after all, we didn’t get rid of our previous car until it had 264,000 miles on it. I figured we could get the minivan to at least 200,000.

But while visiting relatives in Florida, we had the opportunity to buy a used minivan from my sister’s meticulous neighbor for a great price. So we took it. I have to admit, it’s been a huge upgrade. It has all kinds of luxury features that our old minivan didn’t have, such as:

  • When you turn it on, it stays on until you turn it off.
  • When it rains, the water stays out of the vehicle.
  • There’s an electrical device in the cigarette lighter that I can use to charge my cell phone, rather than just an empty hole where an electrical device used to be.
  • The speedometer is an accurate reflection of the speed I’m traveling, rather than a number to which I have to add five to 15 mph.
  • The brake light comes on only when the emergency brake is actually engaged.
  • It has this thing called “air conditioning.”
  • It doesn’t stink. (The smell in the old car came from water coming in and getting the carpets all moldy.)
  • It has a “keyless remote,” which is a device on the key ring with buttons that, when pushed, cause the side doors to slide open.

(Regarding that last feature, here’s a trick you can play on the uninitiated: I put the keyless remote in my pocket, and told my mom and my aunt that the doors were voice activated, but you had to use the secret word. In this van’s case, I told them, the secret word was “monkey _____.” Since this is a family website, I can’t print the actual word, but use your imagination and you’ll be close. So I got my 70-something mom and aunt to yell, “Monkey ____!” at the van, pressed the button in my pocket, and — voila! — the doors opened. They just couldn’t get over it. “I have to get me one of those!” my aunt exclaimed. I let them yell, “Monkey ___!” at the van for another 15 minutes, closing and opening the doors, until I told them the truth. If I had videotaped it, we’d all be YouTube heroes by now.)

Drive a lemon, save some cabbage
I take my share of barbs from family and friends for driving clunkers. But for me, there are two driving (no pun intended) factors: 1) money, and 2) safety. Let’s look at the first one.

At what point is repairing an old vehicle just throwing good money after bad? Here’s one way to look at it: According to Comerica Bank, creator of the Auto Affordability Index, the average cost of a light vehicle last year was $25,500. If you had to finance that total amount over four years at the going rate of 6.56% (according to Bankrate), it would cost you $605.44 a month. If you spend less than that on repairs of the old vehicle, then keeping it saves money…at least for the next four years. At that point, the newer car would be paid off, and the old car would be really old.

But in the meantime, if it cost you only $200 a month to maintain the old vehicle, you saved $405.44 a month, or $19,461.12 over four years. Not too shabby.

While that’s helpful as a way to begin to think about the question, it doesn’t really help with determining a bottom-line number for an actual person because it’s based on an average number, 100% financing, and the purchase of a new car. To help someone like you — who is non-average, who likely wouldn’t finance the entire purchase, and who might be buying a “pre-owned” car (which, to me, is an inappropriate term, since “pre-owned” should mean “before it was owned,” that is, new) — you’ll have to whip out a spreadsheet and factor in all the costs. That includes assuming higher maintenance costs for the older car, but also higher insurance, higher property tax (if you, like me, live in a state that charges such things), and transaction costs after replacing your jalopy. It’s not an exact science, since it involves forecasting future maintenance costs — an impossible task. But it will give you some numbers to think about. In most cases, I bet the analysis will show that keeping the old car is the way to go, financially.

But then there’s safety
Did I do just such an analysis when I decided to replace our old minivan? Not exactly. We had reached a point where we were spending more than $2,000 a year on maintenance, and I had a rough idea of how much it would cost to fix the major problems with our van. Then this opportunity came up, to buy a van from one of those people who perform regular maintenance and have the records to prove it, at a price $2,000 to $5,000 below what it would have cost me to get the same vehicle from Craigslist or Carmax.

But what really got me was that the old van just didn’t feel safe anymore. Something was always going wrong, and I didn’t want something major to happen while my wife was on the highway with our kids. And I must admit, it’s nice to have a car that stays dry, has A/C, and doesn’t stink…yet. If I ran the numbers, I suspect my spreadsheet would have said, “Keep the old car.” But as readers of this website know, financial decisions don’t always come down to dollar and cents.

So I feel comfortable with the decision we made. As for our old van, my sister and brother-in-law got the major problems fixed and arranged to have it donated to a church they work with, so we’ll get a tax deduction out of the deal (another factor that would have gone into the spreadsheet) and the van will hopefully last long enough to do some good. My relatives in Florida approved of the trade-off, especially those who make jokes about the heaps we’ve driven, and believe that any car should be replaced once it reaches 100,000 miles.

Yet the math clearly shows that driving a car well beyond that point is the right thing to do for your net worth. According to Comerica Bank, the purchase of a new car costs 21.9 weeks of a median household’s income. That’s working 42% of the year just for a car. To me, acquiring a new vehicle before it’s necessary is a bunch of financial monkey _____.

This article is about Cars, Choices  Wednesday, 3rd February 2010 (by J.D. Roth)