Monday, May 6, 2019

Five Years of Driving

It’s been five years since I bought my Subaru. Time for an assessment.

The odometer now reads 12,575, so I’ve driven the car about 2500 miles per year. To most Americans that won’t sound like much, but it would be a long way to walk, and it’s about twice the mileage I put on my bicycle.

Today’s cars are made to be driven hundreds of thousands of miles, so I feel kinda ridiculous for investing $25k in a new one and then using it so much less than I could. It seemed like the best of several bad options at the time, and I still can’t really think of a better one.

Unsurprisingly, the car has been virtually trouble-free. I get its oil changed once a year whether it needs it or not. The battery ran low a couple of times this last winter, while the car sat in the driveway unused for weeks at a time. The only other service it’s needed was also due to lack of use: a warranty-covered replacement of the fuel line vent valve, which had gotten clogged with spider webs.

I do nearly all of my commuting, grocery shopping, and other short errands by bicycle, so generally I use the car around town only when I need to carry a passenger or some other large cargo. Most of the miles on the car are from recreational trips: up into the mountains to hike or to ski, plus a couple of trips each year to neighboring states. It’s never been farther from home than northwestern New Mexico.

I chose a Subaru Crosstrek for its high clearance, and I’ve taken it a few places where high clearance was necessary, but only a few. I’m conflicted over whether those few trips were worth the added expense and/or added carbon emissions, compared to (say) a low-clearance economy hatchback.

Fuel economy

So far I’ve filled the car’s gas tank 31 times, for a total of 412 gallons. At the last fill-up the mileage was 12,218, so the overall fuel economy comes to 29.6 miles per gallon. Here is a chart that shows the variability from one fill-up to the next:

As expected, the best fuel economy has been on summer road trips, while the worst has been in winter city driving. But this tank-by-tank data doesn’t provide the precision one might like, because the tank size is pretty generous and I typically drive about 400 miles before each refill. Except on long trips, those 400 miles always include quite a mix of driving conditions.

In principle I could get more detailed information from the dashboard fuel economy display. But care is required, because its calibration is off. As the next chart shows, the displayed fuel economy is higher than the calculated-at-the-pump fuel economy by an average of 2.6 mpg:

With this calibration inaccuracy in mind, I’ll report that on one occasion—a round trip from Ogden to Salt Lake City in September 2014—the dashboard reported a fuel economy as high as 40.8 mpg.  I’ve seen higher numbers only for one-way partial trips that were mostly downhill.

My car’s official EPA-estimated fuel economy is 25 mpg in the city, 32 on the highway, and 28 overall. So I’ve been doing slightly better than the official estimate. That’s mostly because I do proportionally more highway driving than the EPA assumes, and very little of my highway driving is at the absurdly wasteful (and dangerous!) freeway speeds that Utah allows.

Carbon footprint

Burning a gallon of gasoline produces just under 20 pounds of carbon dioxide, so at 2500 miles per year and 30 miles per gallon, my Subaru has been emitting roughly 20×2500/30 = 1660 pounds of CO2 per year, or 0.75 metric tons. The EPA estimates that upstream emissions from producing and transporting the gasoline add on another 24 to 31 percent, so my car’s annual carbon footprint is probably about 2100 pounds or 0.95 tons of CO2. (This doesn’t include the substantial emissions from manufacturing the car in the first place.)

My personal driving-related carbon footprint isn’t the same as my car’s, because the car sometimes carries other passengers and I sometimes travel in other cars. I haven’t kept the records I’d need to determine which of these effects is larger, so let’s just assume they cancel each other out. Then it’s meaningful to compare my Subaru’s carbon footprint to my own carbon emissions via other means, and to national and international per-capita averages.

Even though my carbon footprint from driving is several times smaller than the U.S. average, I don’t feel like I’m sacrificing anything to keep it so small. I’ve always disliked driving, so I’ve always naturally chosen to live within biking distance of where I work, and to just say no to most of the driving opportunities that continually present themselves. It helps that I also dislike shopping. Rarely, on a cold, rainy night, I’ll give in to the temptation to jump in the car when I need some groceries. But as a modern American who sits on his ass indoors most of the time, I rarely want to sit on my ass, wrapped up in a tin can, even when I’m outdoors.

Of course the future of cars is electric, but it’s hard to guess when an electric car might be in my future. Electric cars are best for daily commuting—precisely the type of driving that I never do. Charging stations are still rare to nonexistent along Utah’s two-lane highways, not to mention remote trailheads. Subaru actually just came out with a plug-in hybrid version of the Crosstrek, but its range on battery power is only 17 miles (not even enough for a round trip to the upper Ogden Valley), and it costs an extra $10k. Finally, a full 70% of Utah’s electricity still comes from coal, so there’s little or no CO2 reduction from driving an electric car around here. All these things are bound to change, but that change may take a while.