Here’s an interesting question.

Suppose you had a household with two cars, and each car needs to be driven 10,000 miles per year. One car consumes 34 MPG, and the other car consumes 18 MPG. Since gas is expensive, you want to replace one car. Because of utility constraints, you have two choices:

Which car replacement would save you the most gas?

Normally, I consider myself not bad with quantitative comparisons like this, yet initially I picked the answer of replacing the 34 MPG car with the 50 MPG car based on the superior 16 MPG improvement. Another seemingly more analytical approach also leads to the same conclusion: 50 + 18 MPG giving a 34 MPG household average seems more efficient than 34 + 28 MPG giving a 31 MPG household average.

This very interesting article in Science, “The MPG Illusion” by Richard P. Larrick and Jack B. Soll at the Fuqua School of Business in Duke University (Vol 320, June 20, 2008, p. 1593), points out the mathematically obvious truth that gas used per mile is inversely proportional to miles per gallon, which means that you have a steeper slope at lower MPG ratings, and diminishing returns at higher MPG ratings.

(The above image is taken from the article, available here with subscription).

When you run the numbers, replacing the 34 MPG car with a 50 MPG (a 16 MPG improvement) car saves you 94.1 gallons per 10,000 miles, whereas replacing the 18 MPG car with a 28 MPG (a 10 MPG improvement) car saves you 198.4 gallons per 10,000 miles — more than double the savings.

Or, to give an even more clear-cut example, replacing a 5,000 MPG car with a 10,000 MPG car saves you just one gallon of gas, whereas replacing a 1 MPG car with a 2 MPG car saves you 5,000 gallons of gas, using a fixed mileage of 10,000 miles driven for comparison.

There are some important policy implications of this. Relatively small MPG improvements in the most gas-hungry vehicles pay off greater than larger improvements in already efficient cars (hence, it does make sense to offer tax breaks for modest improvements in SUVs versus tax breaks for hybrids, which typically replacing already gas-efficient sedans). Also, personal driving habits, especially for gas-hungry cars, can often times add or subtract a few MPG to a car’s efficiency on average. For example, a car that may get 25 MPG “average highway” will degrade to under 15 MPG if you gun it out of stoplights in city traffic. That’s a huge increase in gas consumed per distance driven, especially for the less efficient cars, whereas for more efficient cars it doesn’t hurt as much to goose the engine a bit.

Apparently the thinking that gas savings is linear with MPG is not uncommon. A survey of college students revealed that a majority of them shares this misconception. I’m not sure what the sociological term is for such a massively accepted factual inaccuracy, but it seems like a textbook case for how common wisdom can fail the common person. It’s also a good example of why you don’t want to put policies to a vote — people just don’t have the time to run the numbers, and simple numbers can be so simply deceptive, even with the best intentions. Good democracies are probably more about the people directly controlling principles (“conserve oil”), rather than the policies (“reward car makers that achieve the greatest fleet MPG delta”). Of course, that doesn’t address the problem of creating accountability between principles, policies, and politicians that make the policies to execute the principles.

Keeping in line with the exit review series, I’ve written an exit review of my IBM T60p laptop, which was recently retired.

The T60p was purchased back in March 2006, right when the first dual-core mobile chips started shipping. The IBM model number was 2623-DDU, and it featured an Intel T2500 (2GHz), 2GB RAM, 100GB 7200rpm HD, 15″ 1600×1200 LCD, 512MB ATI FireGL V5200, CDRW/DVDRW, Intel 802.11abg wireless, WWAN, Bluetooth/Modem, 1Gb Ethernet, UltraNav, Secure chip, Fingerprint reader, 9 cell Li-Ion batt, and WinXP Pro. Like most of my key gadgets, the T60p followed me everywhere around the world; I spend probably 16 hours a day with it, so it got plenty of wear and tear.

I retired my T60p because Microsoft is retiring Windows XP, and I really don’t want to downgrade to Vista (“congratulations, your new faster hardware now runs slower than ever thanks to Vista!”), so I decided to get the latest, greatest Windows XP laptop available today, and hunker down and hope it lasts me until Microsoft figures out how to get Vista right (or they realize what a terrible mistake they’ve made and bring back XP…sort of like how Intel brought out the Pentium 4 but now all the new designs are Pentium III derivatives). Or maybe before that, Linux support for all of my key apps will get so good, I can finally depart from the Windows ecosystem.

So the reason for getting rid of the T60p isn’t because it had any functional flaw; it’s actually in fair working order, and its performance is still decent.

Here were some of my favorite features about the T60p:

Well, that’s my experience with the IBM T60p. I think my experience can be summarized by the fact that my new laptop is an IBM T61p — they won a repeat customer. The T61p is still settling in. I like the extra performance it brings, but it has some quirks of its own that I’m still learning to get used to. It’s funny, but every time I get a new laptop, I have to learn all of its little bugs and find ways to patch my behavior to avoid aggravating them. For example, I’ve trained myself to unplug my webcam before hitting the eject button on the minidock, because failing to do so causes a blue screen without fail. So far, none of the quirks have been so bad that I’ve given up on the machine. In addition to the webcam issue, I did have a spate of blue screens to deal with when I first got the laptop (mostly around minidock ejection and going into and out of standby), and I had to do some digging around in the Minidump logs to figure out exactly what behavior was triggering them. I wonder what other users do when this sort of thing happens to them?