The ramblings of a giant squid…

Gas is cheaper across town?

Friends-Romans-Countrymen, Math and Science, Travel

This is a rewrite of an old article that was lost, but it bears repeating (and it’s been 8 years since the old article anyway).

We see all the time articles about where to get the cheapest gas.  Plenty of people will drive all over Hell’s half-acre to save a few pennies on gasoline… or so they think.

It may surprise you to learn that, in general, it’s not really worth driving out of your way to buy cheap gas unless the price difference is truly huge, and through the magic of mathematics, this can be shown.

Starting conditions

I’ll consider 4 cars in this article:  2014 Nissan Rogue, 2014 Ford Focus, 2014 Toyota Prius, 2015 Dodge Caravan.  I’ll be using stats/numbers from

The rated city gas mileage, and fuel tank size for these vehicles is:

  • Rogue: 23 MPG, 15.9 gal (US) / 10.2 l/100km, 60.4 l
  • Focus: 26 MPG, 12.4 gal (US) / 9.0 l/100km, 47.1 l
  • Prius: 53 MPG, 9.5 gal (US) / 4.4 l/100km, 36.1 l
  • Caravan: 17 MPG, 20.0 gal (US) / 13.8 l/100km, 76.0 l

With a gas price hovering around $1.10 per litre, you can calculate how much it costs to drive 1 km in any of those vehicles:

  • Rogue: 0.10 l for 11 cents
  • Focus: 0.09 l for 9.9 cents
  • Prius: 0.04 l for 4.4 cents
  • Caravan: 0.14 l for 15.4 cents

Now most people fill up around 1/4 of a tank, so that means for each of these vehicles, you’re buying (more or less) a known amount of fuel:

  • Rogue: 45.3 l
  • Focus: 35.3 l
  • Prius: 27.1 l
  • Caravan: 57.0 l

This, in turn, means that for every $0.01 per litre lowering in price, you save the listed amount of cents in the total purchase.  It also means you can calculate the maximum distance to drive to buy cheap gas before you’re simply wasting money.

The Math

Assuming you have to drive to the gas station and back to where you started, that effectively cuts the distance you can drive to the gas station in half (x there and x back).  So if you are buying 3/4 of a tank of gas with those vehicles, the following calculation results:

  • Rogue: 45.3 l = $0.453 savings at 11 cents per km = about 4 km round trip
  • Focus: 35.3 l = $0.353 savings at 9 cents per km = a bit less than 4 km round trip
  • Prius: 27.1 l = $0.271 savings at 4.4 cents per km = a bit over 6 km round trip
  • Caravan: 57.0 l = $0.57 savings at 14 cents per km = a bit over 4 km round trip

In short, you get about 2 km distance to the gas station for every cent per litre cheaper just to break even, so you need to come in under that distance to actually save money, not counting your time and effort involved.

In my neighbourhood, that means buying gas at the cheapest gas station, but it also means not actually driving very far outside of my local area.

Here’s a good example of false economy… I might be inclined to buy gas at Costco – they’re usually about $0.05/l cheaper, but the nearest Costco station is about 7.7 km from my house (according to Google Maps) for a round trip of 15.4 km, versus the nearest gas station (Quickie, 1.7 km = 3.4 km round trip).  The difference (12 km) is paid for in gas I have to spend to get the savings.

Since I drive a Rogue, I’ve got the potential to save $2.26 by going to Costco, but I’ll spend $1.32 to do it.  I come out to $0.94 to the good by going to Costco, but I’ve blown about 30-60 minutes of my time to do it depending on traffic (yeah, it’s a bit of a nightmare where I live) and the line at the pump.  Since I fill up twice a month, I can waste up to 2 hours of my time each month for a saving of less than $2.  Most people, if offered $1.88 for an hour or two of their time would laugh.  Nevertheless, there’s a potential saving of almost $24 per year.  In my estimation, that’s just not worth it.  I’m happy to pay the extra cents for the convenience.

If one happens to be going that way anyway (i.e. need to buy gas and you’re passing a selection of stations) it makes sense to choose the cheapest one, but I can’t really see how it’s worth it to drive very far out of your way unless the gas is super, awesomely cheaper.

By way of comparison, the second closest gas station to my home is 5.6 km round trip, so it’s barely worth going to that station over the closest one…

Interestingly the Prius hybrid (because it sips gas) and the guzzling mini-van (because it has a huge tank so you’ll be buying lots of gas) offer the best opportunities for driving far and wide to seek cheap gas.  With the little Ford Focus, you simply can’t buy as much gas to realize the savings.

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  • MichaelWH on 2014-11-25

    But … math is hard! 😀

  • squid on 2014-11-25

    I’ll be hitting a class with a slide of Maxwell’s equations tomorrow…

  • squid on 2014-11-30

    At Costco today people were lined up 9 deep for $0.99 / l. Even at 3 minutes a full that’s just about a half hour wait. Ugh.

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