Charlie posted a link to some controversial advice that it’s better to keep your car rather than get something more fuel efficient.

The advice makes a lot of sense when you do the math.I bought a fuel-efficient car in 2003. I had no choice; I had to buy something. My lease was up and I wasn’t going to buy that car. So I figured I should buy something really fuel efficient and reliable, so I bought a year-old Honda Civic. I’m still driving it today and it’s been a great car.

Now let’s say I’d bought something less fuel efficient, like a Hummer H2, that gets 14 MPG. If I drove that to work every day, I’d burn 29 gallons of fuel per week, and I’d be hurting. With gas at $3.53 a gallon right now, that would be $102.37 per week, which is ridiculous.

So let’s say I had my eye on a 2005 Hybrid Honda Civic priced at $17,000. It gets about 45 MPG. So it would burn a reasonable 9 gallons per week, at a cost of $31.77. The car would save me a cool 70 bucks a week.

It would only take 243 weeks for that car to pay for itself. In other words, not quite five years.

Of course if I traded in the Hummer, I wouldn’t have to pay the full $17,000. If I could manage to sell that H2 for the $18,000 a used Hummer H2 is supposedly worth, then it would make sense to do it. If I could get $10,000 in trade for it, it would take two years for the hybrid to pay for itself. That’s still worth doing, but it’s probably longer than you would expect.

This is proof that buying a car is often an emotional decision.

To make it a logical decision, you need to figure out what you’re spending in gas per week, then figure out what you’d spend driving something else. Subtract the difference, then divide the cost of the car by that savings.

This was a big reason why I bought a conventional Civic rather than some kind of hybrid back in 2003. I would have paid about a $7,000 premium for the hybrid. It probably would have paid for itself, barely, by now. But at the time I made the decision, gas cost less than half what it costs now.

In some cases, it would make sense to switch. But you have to be near the extremes (such as from Hummer to hybrid) to do it. The further you get from that kind of extreme, the less sense it starts to make.

Don’t let a salesman or what the neighbors say sway your decision. Do the math.