Wow! A Big, Round Number!

I saw this sign when I was filling up my tank and thought “700 million gallons a year, eh? Sounds like a big deal. I wonder if it really is.”

Let’s do a few “back of the envelope” calculations and see what we get.

First off, let’s try to get a feel for how much “stuff” 700 million gallons of gas is. It would fill more than 2 Houston Astrodomes, 42 Spaceship Earths at the Epcot Center, or an average twelve-mile stretch of the Erie Barge Canal.

700,000,000 gallons would weigh as much as nearly six Empire State Buildings, it would take a full 15 minutes to flow all of this fuel over Niagara Falls, and It would cost about two billion dollars to buy this much gas at an average gas station (assuming you had the 190 years it would take to “fill ‘er up” at an average pump).

[Insert impressed whistle here] Sure does sound like whole big bunches… Let’s look at some more numbers, though.

WolframAlpha (always with the WolframAlpha, this guy… :)) says that there are 254.4 million registered vehicles out there.

The USDOT and other sources say that these cars drove about three trillion miles altogether last year, so everyone seems to drive about 11,800 miles annually. Meh, sounds low. 12k-15k seems to be the range using various sources out there, but it is in the ballpark.

Assuming everyone’s tires are equally deflated (so everyone shares blame) each car wastes 2.75 gallons per year. Since the average fuel efficiency for these vehicles is around 25 miles per gallon, these cars “lose” 68.8 miles of driving potential each year. I can only assume people unconsciously budget for more gas to cover this loss rather than shrug their shoulders and stay home or hoof it for the rest of the year when their tank goes dry on December 28th or so…

68.8 miles is about 0.6% of the distance the average person drives (or, in other words, is the hit their fuel economy takes) each year. According to the fueleconomy.gov website “Under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires”. Ok. This implies that everyone’s tires are, on average, two pounds low, or about seven percent.

Hmmm. This is sounding less and less like a big deal, but, I am a worrier so let’s see if maybe there is something I can do about this apparent epidemic that needed to be brought to my attention in bright yellow 100-point font.

Let me whip out my tire gauge and just… Wait. How accurate is this thing, anyway? If I am going to be able to help save gas I need a reasonably precise measurement of how much air is in my tires. According to this Edmunds site, the very best you can realistically expect from your average (mechanical) pressure gauge is +/- 2 pounds from true, which totally swamps the estimated underinflation you are looking for.

But it gets better. The temperature in an average American city typically fluctuates about 20F degrees over the course of a day. According to tiretrack, a good rule of thumb is that tire pressure will vary about 1 pound per 10F degrees, so that’s another two-pound delta right there.

Of course, the temperature of your tires (and, therefore, the pressure) increases as the rolling friction from driving heats them up. The temperature of the road you are driving on and whether it is wet or dry matters, too.

But let’s say for the sake of argument that you have a hyper-accurate pressure gauge and you are measuring your tire pressure in your hermetically sealed, temperature controlled garage just before you drive the car anywhere. How often should you measure your tires to save money?

Well, as of the writing of this post, the average cost of a gallon of gasoline is $2.88, meaning that you waste about eight dollars per year due to your shameful, shameful negligence of your tire pressure. Let’s say that your name is Joe Q. Pitcrew and it takes you three minutes to check and correct all four tires on your car. If you check it every day, you will have spent 18.25 hours over the course of a year fiddling around like this.

Since we are playing with averages, here, the mean wage for a worker in the USA is $43,460 per year, making the average person’s time “worth” $20.89 per hour. Checking your tires every day, then, could be seen as costing you $381.24 (in time) per year! To break even, assuming your tire gauge and air compressor usage are both free, the most often you should check your tires (assuming the very act of checking magically puts them in stasis until the next time you check) is about 7 or 8 times a year, or about every month and a half.

Seven or eight times a year seems reasonable – high, but not excessive. To tell you the truth, though, I am probably just going to continue with my time-tested method of “sometimes-noticing-that-one-of-my-tires-seems-low-if-I-happen-to-be-approaching-the-car-from-that-direction-and-I-am-not-distracted-by-anything-else”.

To be clear, I am not advocating driving around on your rims all grindy and sparky and stuff. I am just saying that ExxonMobil probably should have taken the money they spent on advertising just how much they care about my tires (and, by extension, Planet Earth, I guess) and donated it to charity or lowered the price of gas a smidge or something.

Just sayin'.