Geek Out
Yesterday, on my way to church, I noticed my gas gauge was showing less than 1/8 of a tank. Usually, I’m a stickler for refilling at ¼ of a tank. However, I was anxious to get home after church, so I decided to get gas this morning on my way to an appointment.
I’m a worrier, so I changed my truck’s display from the speedometer to the gas mileage display, which shows average miles per gallon, an estimate of miles until the tank is empty and the instantaneous miles per gallon. With my mathematical training, I understood these calculations and their limitations. So, I thought I’d geek out at them.
Let’s start with the average miles per gallon (mpg). The computer tracks miles driven and divides them by the gallons of gas used to calculate miles per gallon. If you’ve driven 30 miles and used 2 gallons of gas, your average is 15 mpg.
The next easiest is estimating the distance to empty. If the average is 15 mpg and you have 10 gallons of gas remaining, you should be able to travel 150 miles before your tank is empty.
The last display item is the instantaneous mpg. If you can track miles driven and gallons used, then you can use any interval of miles and gallons to track mpg. For example, if you went half a mile and used 1/30 of a gallon of gasoline, your average is 15 mpg. If you went 1/10 of a mile and used 1/150 of a gallon of gas, your average is 15 mpg. Obviously, I’m using easier numbers to calculate this, whereas the computer can use infinitely small changes to calculate an instantaneous mpg.
Here is where knowing the limitations of the display numbers can help you avoid a big problem. The average mpg is always based on the miles driven and the gallons used since the last reset. With my driving habits and usual routes, my truck averages about 15 mpg. However, when I’m towing a trailer, I’m lucky to get 8 mpg. In fact, if you watch the instantaneous mpg, you can see it reach 99 mpg, which is as high as the computer can calculate, coasting down a hill and low single digits when I’m accelerating from a stop. As the terrain, acceleration and speed change, the instantaneous mpg changes. Only the average over a longer time for a specific vehicle, driver and area makes an average mpg consistent.
Many years ago, we were towing our trailer in southeastern Kansas. We started with about half a tank of gas and planned to stop in southeastern Kansas and fill up on the way. I breached my safe refill point (1/4 tank remaining) and my time-to-fill-now point (1/8 tank remaining), so I started looking for a gas station. We had passed a small station earlier in the day and I was ready to turn around and go back. My wife asked what the miles-until-empty estimate was and calculated that we should be able to make Pittsburg (where she knew of gas stations) with that estimate. About 10 miles away from Pittsburg, my truck ran out of gas. As we waited for the motor club to bring us a couple of gallons of gas, we tried to figure out why we didn’t make Pittsburg. We finally figured out that most of the gas usage in the average mpg calculation was before the trailer was hooked up for the trip. Had I reset the mpg computer after hooking up the trailer, it would have used the trailer mpg, which averages about half of my non-trailer mpg, and estimated I could only go about half the miles until empty.
Most people think of mpg as a constant thing: you have 10 gallons left and you have gotten 15 mpg on average; you should be able to go 150 miles. That only works if the future miles are the same as the previous miles. If some aspect changes–different driver, different vehicle, different terrain–the actual result may vary from the estimate.
The same thing happens in life, with many estimates being derived from experience. For example, we hear all the time about the inflation rate. At the time I’m writing this, it’s running about 4.2%. This does not mean something that cost $100 last year will cost $104.20 this year. Some items might increase 20% while others may decrease in cost. But, given a fixed basket of purchases, it went from $100 to $104.20 over the last year. Even more problematic, just because that fixed basket of purchases increased by 4.2% over the last year doesn’t mean it will go up by another 4.2% in the next year.
“And the merchants of the earth weep and mourn for her, since no one buys their cargo anymore, cargo of gold, silver, jewels, pearls, fine linen, purple cloth, silk, scarlet cloth, all kinds of scented wood, all kinds of articles of ivory, all kinds of articles of costly wood, bronze, iron and marble, cinnamon, spice, incense, myrrh, frankincense, wine, oil, fine flour, wheat, cattle and sheep, horses and chariots, and slaves, that is, human souls. “The fruit for which your soul longed has gone from you, and all your delicacies and your splendors are lost to you, never to be found again!” The merchants of these wares, who gained wealth from her, will stand far off, in fear of her torment, weeping and mourning aloud, “Alas, alas, for the great city that was clothed in fine linen, in purple and scarlet, adorned with gold, with jewels, and with pearls! For in a single hour all this wealth has been laid waste.” And all shipmasters and seafaring men, sailors and all whose trade is on the sea, stood far off and cried out as they saw the smoke of her burning, “What city was like the great city?” And they threw dust on their heads as they wept and mourned, crying out, “Alas, alas, for the great city where all who had ships at sea grew rich by her wealth! For in a single hour she has been laid waste. Rejoice over her, O heaven, and you saints and apostles and prophets, for God has given judgment for you against her!”” - Revelation 18:11-20 ESV
We all long for a world where all the inequities of our fallen world are gone. A world where we don’t have to wonder whether we have enough to be comfortable. We need to remind ourselves of what our Lord told us.
“Therefore I tell you, do not be anxious about your life, what you will eat or what you will drink, nor about your body, what you will put on. Is not life more than food, and the body more than clothing? Look at the birds of the air: they neither sow nor reap nor gather into barns, and yet your heavenly Father feeds them. Are you not of more value than they? And which of you by being anxious can add a single hour to his span of life? And why are you anxious about clothing? Consider the lilies of the field, how they grow: they neither toil nor spin, yet I tell you, even Solomon in all his glory was not arrayed like one of these. But if God so clothes the grass of the field, which today is alive and tomorrow is thrown into the oven, will he not much more clothe you, O you of little faith? Therefore do not be anxious, saying, ‘What shall we eat?’ or ‘What shall we drink?’ or ‘What shall we wear?’ For the Gentiles seek after all these things, and your heavenly Father knows that you need them all. But seek first the kingdom of God and his righteousness, and all these things will be added to you. “Therefore do not be anxious about tomorrow, for tomorrow will be anxious for itself. Sufficient for the day is its own trouble.” - Matthew 6:25-34
