Running on Empty

I read somewhere once that carrying around 100 lbs of dead weight in your car reduces its fuel efficiency by about 1%.

So if you drive 15,000 miles per year and get 25 mpg, then you’ll use about 600 gallons of gas each year. With an extra 100 pounds reducing your fuel efficiency, it would take about 606 gallons to go the same distance. If gas is $3.30/gallon, then you’re paying about $20 a year just to carry that weight around.

It occured to me one time I was pumping gas that a typical car’s gas tank holds about 15 gallons and gasoline weighs a little over 6 pounds per gallon, so a full gas tank should weigh about 100 pounds more than an empty one.

Normally, I fill up the tank completely each time, even pumping a few times extra after the pump has shut off. But this means that, immediately afterwards I’m toting around a full tank of gas, reducing the car’s fuel efficiency as a result.

How much money could we all save by just filling our tanks to the halfway point instead? It would mean pumping gas more often, but by eliminating 50 pounds of dead weight on average, should save about 3 gallons per car per year.

Taking 606 gallons in 15 gallon increments means visiting the gas station about 40 times per year. But taking 603 gallons in 7.5 gallon increments means pumping gas about 80 times.

If each visit takes 10 minutes, then it would take 400 minutes (about 6.7 hours) each year to make those 40 extra visits and save that $10.

This means that the extra time spent pumping gas would save me about $1.50/hour, tax-free.

I am not certain that only filling my tank to the half-way point is justified by this level of savings, but if filling the tank half-way only took 5 minutes per visit and if gas prices were double or triple their current prices, then this strategy might begin to make sense.

There’s no real point to this post. This is just the sort of stuff I think about while pumping gas.


  1. Bradd, the marginal value of most people’s time, at least in the U.S., is clearly greater than $1.50/hr. Thanks for figuring this out though, I have often wondered the same thing standing there pumping my gas. If gas triples you should definitely revisit this.

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