SUV or Hybrid? A Familial, MPG Conundrum

Jan 31, 2005

RAY: This puzzler came to us from Rabbi Jeff Glickman. Here it is:
You drive a gas-guzzling SUV that gets a whopping 10 miles per gallon. Your sleek and efficient spouse drives a sleek and efficient hybrid that gets 100 miles per gallon.

Now, let's assume that both of you drive the same distance each year. Your spouse sees an ad for a new, super-duper hybrid that gets 200 miles per gallon. She is lobbying to trade in her old, wasteful 100-mile per gallon hybrid for the new 200-mpg model; her thinking being that getting this new hybrid to replace the old one will really improve the average miles per gallon of your household.

On the horns of a dilemma, you seek out the one person you trust with questions automotive: your mechanic, Crusty!

"What can I do," you ask, "to improve our household's miles per gallon without buying this new hybrid?"

Crusty says, "Well, if we tune up your old SUV, and inflate the tires correctly, and remove that four inches of accumulated bird poop on the roof, I'm pretty sure we can get you all the way up to 11 miles per gallon." Your heart sinks.

What should you do? That's the question. Under which scenario would your household see the biggest improvement in miles per gallon: by getting the new hybrid, or by tuning up the old SUV?
RAY: The answer isn't intuitive. Let's say you drive 200 miles with your tuned-up SUV. So, instead of getting 10 miles a gallon you're getting 11 miles per gallon, right? And that'll use up about 18 gallons of gas.

Now, if you kept the old hybrid to drive that same 200 miles you're going to use how many gallons of gas, two, right? So you've got 18 and 2, that's 20 gallons of gas, are you with me?

TOM: I'm with you.

RAY: For the same 200 miles driven, if you buy the new super-duper hybrid, you're going to use one gallon of gas to drive 200 miles.

TOM: So you're going to use one gallon of gas.

RAY: Right. One measly gallon. But because you didn't tune up the SUV, you're going to use 20 gallons at 10 miles a gallon. So instead of using 20 gallons of gasoline you're going to use 21 gallons of gasoline even with the brand-new hybrid. It's hard to believe. Do we have a winner?

TOM: What a great SAT question. Yes, we do have a winner. It's David Riley from Caldwell, Idaho. And for having his answer selected at random, David, you're going to get a 26 dollar gift certificate to the Shameless Commerce Division at where you can also get a brand-new stainless steel Car Talk thermos.

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