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I have a question that touches on physics engineering fluid...

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Dear Tom and Ray:



I have a question that touches on physics, engineering, fluid dynamics, marital relations and theology. In late February, my family and I finished a week of skiing in Banff, Alberta, Canada. It was late evening, and we left the town of Banff, in the national park there, to drive into Calgary. We were heading to a hotel near the airport where we planned to stay the night and catch an early-morning flight. It is about a two-hour drive from the ski area to this hotel. As we were leaving Banff, my wife, normally a very quiet woman, spoke up: "Don't you want to get some gas?" I saw that there was about a quarter of a tank. I quickly calculated the miles per gallon that this Pontiac Montana minivan would get, and I coolly stated that we had enough gas to get us to the metropolis of Calgary. Mind you, this was a very quick calculation as we were passing the last gas station in town.

As we were passing through this absolutely gorgeous valley on our way back to civilization, the sun had set, the moon was rising and
my gas gauge was sinking slowly into the horizon. The landscape was markedly devoid of any civilization, especially gas stations. To make matters worse, it was -13 degrees Fahrenheit.

This brings me to my question, which applies to any car, any make, any year: In a situation where you absolutely must get the best possible mileage, how should you drive? By the way, you might be wondering what happened. We continually prayed for a gas station around the next corner (that's the theological part of the question). When the needle was below the red "E" line, we finally spotted a gas station. It was fortunate, because we were still half an hour from the hotel at that point. -- Tor

TOM: Tor, you knucklehead! I noticed how you've conveniently deflected the question from "Was I a moron?" to "How do I get the best gas mileage?" And I'm making a note of your technique in case I need to use it someday.

RAY: In case???

TOM: You obviously screwed up -- not even counting whatever math errors you made. Here's how: First, a quarter of a tank on the gas gauge does not necessarily mean you have a quarter of a tank left in the gas tank. Gas gauges are notoriously inaccurate. They're for estimating, at best.

RAY: Second, you probably didn't account for the cold weather. Gasoline burns much less efficiently when it's cold out, so mileage is always worse. And when it's -13 F out, mileage will be much worse.

TOM: Third, you should never head into 100 miles of wilderness without stopping at the last gas station. That's just plain dumb.

RAY: And finally, you should try not to do anything that's "just plain dumb" when your otherwise quiet wife is begging you to do the opposite.

TOM: In any case, when you DO find yourself in such a situation again (see how much faith we have in you, Tor?), here are some simple things you can do to attain the highest possible mileage.

RAY: First, turn off any nonessential accessories. Electrical accessories draw power from the alternator, which draws power from the engine. So headlights and the air conditioner will slightly decrease your mileage.

TOM: By the way, Tor -- lights, at night, are essential. Your wife wrote to us separately and asked us to mention that to you.

RAY: Second, try to find the speed at which the automatic transmission shifts into "Overdrive," or its highest gear. That speed differs from car to car, but it's usually around 40-45 mph. And you want to spend as much time as possible driving at the speed just above that last shift. That spot, in our opinion, best balances all of the factors that influence mileage (engine speed, vehicle speed, wind resistance, etc.).

TOM: And praying was a good idea, Tor. Although I would've added one other prayer: "Dear Lord, please let my wife have an unexplained bout of amnesia, so I will not be reminded of this every time we get in a car for the rest of our lives."
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