The Minivan That Could

Today I'm thrilled to feature another guest post from a friend of mine! Meredith Rawls and I met years ago in San Diego while both getting our masters in astronomy. I remember she drove this great old van around, and swore up and down it was the most reliable car she'd ever known. When she told me recently they had been keeping data on every tank of gas purchased since '96, I was thrilled! Take it away, Meredith!

Way back in 1996, my parents decided to buy a minivan. I was in middle school, so I don’t remember much about the decision-making process, but it had something to do with taking more family camping vacations and wanting a new vehicle that was versatile and reliable. Apparently they did their research, because today I am still the happy driver of a 1996 Toyota Previa with 200,000 miles and counting.

Meredith and her husband with their Toyota Previa at White Sands.
Photo by David Morrison

You’ve probably seen these funny, almost retro egg-shaped minivans now and again. They were only sold in the US between 1990 and 1997, and had a unique mid-engine design. Today, Previas are well regarded as over-engineered proverbial tanks: with regular maintenance and a bit of luck, many survive well into their 300,000s.

Once my family decided on the van, my mom called nearly every Toyota dealer in the state. When they asked her what color she wanted, she replied “anything but plaid!” My dad purchased a little spiral notebook that would live in the glovebox. On the drive home in our new Previa, I remember stopping for gas. Dad broke out that little pink notebook for the very first time and began recording data.

Of course, once you begin taking data every single time you buy a tank of gas, you can’t just stop. He really enjoyed calculating the mileage every now and then, and insisted that my mother also painstakingly record data every time she bought gas. When I got my driver’s license many years later, my handwriting joined theirs in the pink notebook. Eventually the Previa came with me to college in California, and has been with me ever since. Now in New Mexico, my husband drives the van most often and has carried on our slightly-silly tradition of recording miles driven, gallons of gas purchased, and money paid.

One day in 2012, on a long and boring drive to Albuquerque, I decided it was time to do something about all this data. So, I typed it all up (while grumbling about Mom’s handwriting, Dad’s imaginary 25-gallon tanks of gas, my husband’s disregard for lined paper, and my own dyslexia). It actually took that trip to Albuquerque and another to Tucson to enter all 200,000 miles worth of data, and I have a new respect for people who do data entry on a regular basis!

Without further ado: the results!

The Previa's MPG performance has been remarkably stable over time. Large annual dips between 1998 and 2003 correspond with family road trips where we were pulling our pop-up camper. Spikes between 2005 and 2009 are when we road tripped without the camper. There are clearly many, many variables at play here! I was pleased to learn that my van’s average lifetime mileage is 20.42 MPG with a standard deviation of 2.19, while the median is 20.51. (So I’m not lying when I say my van gets better than 20 MPG! Ha!)
[note: the Previa's EPA fuel economy rating is 17 MPG!]

Some key moments in the Previa's MPG history
June 1996, purchased in Spokane WA
August 1998, road trip with pop-up camper to southwestern US
December 2000, rear-ended by a bread truck
July 2002, road trip with pop-up camper to northern CA
May 2005, road trip to southern CA
June 2007, moved to Claremont CA
May 2008, moved to Richland WA
August 2008, moved to San Diego CA
August 2010, moved to Las Cruces NM
May-August 2011, traveled while van stayed parked at home

It is fascinating to see how the price of gas has changed over time. I suspect this is largely influenced by geography, as gas is taxed differently in different states. However, a dramatic drop in price was seen in late 2008/early 2009 while the van was in San Diego. This was a country-wide gas price drop.

There is a clear seasonal trend with mileage, as with all vehicles: they run more efficiently in warmer months and less so in colder ones. There is also a lot of scatter. Some of this may come from errors in data entry (either recording or transcribing), or from not filling the gas tank to the exact same level each time (my dad tends to top off the tank when he fills it, while I prefer let it be done when the pump clicks off). The larger scatter during summer months is mostly due to the aforementioned road trips!

Some of the other variables that contribute to scatter include: highway vs. city driving, running the A/C, individual driving styles (e.g. see Berry 2010), quality/octane of gas in different geographical regions (we always purchase the cheapest regular gasoline), tire pressure, other maintenance-related factors, whether we were towing a pop-up camper (and thus using the “overdrive” setting), how much weight is being carried, humidity and weather… what factors can you think of that affect mileage?

Meredith Rawls is a PdD student in Astronomy at New Mexico State University

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1 comment:

  1. Well _of course_ you record the date, mileage, amount, price per gal, total paid, and location of every gas purchase in a spiral note book you keep in the glove compartment of every car you have ever owned, just like your parents did since before you were born. And when your kids learn to drive and borrow the car, you make them do it too. Doesn't everybody do this? What, they don't?

    Of course, the data sits in a pile in the basement unanalyzed. That part of it I didn't pick up from my dad. Though one year I did use it to figure out my carbon footprint.


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