Jeff Epler's blog

Seven Years of Prius (2013) Fuel Efficiency

At the tail end of 2019 I changed to a work from home job, and of course COVID ended most activities outside the house for the bulk of 2020, leading to such low mileage. Fuel economy went up a bit.

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Six Years of Prius (2013) Fuel Efficiency

Our total distance traveled was lower again this year, as was the fuel economy. Once again, I'm disappointed with the overall fuel economy--It's the lowest number of miles, but the third from the lowest number of gallons consumed.

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Five Years of Prius (2013) Fuel Efficiency

Our total distance travelled was lower this year, mostly because our longest driving trip was taken in a rental car. The fuel economy was quite a bit lower, probably because the longer trip we did take was in the mountains of colorado, and a long climbing drive can be pretty awful for efficiency. The car also had its tires replaced this year, and the new tires may not reach the same level of low rolling resistance. Finally, it might be the case that an aging traction battery is affecting performance. Frankly, I'm disappointed with the numbers for the year.

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Four years of Prius (2013) fuel efficiency

Fuel economy and total mileage both went up in 2017 compared to earlier years. However, due to taking more road trips total fuel usage went up.

This year had my best mileage on a single nontrivial trip, shown as 99.9 MPG on 58.5 miles. This drive included significant downhill portions (descending from 12,000 feet to 6000 feet, if I recall correctly).

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Three years of Prius (2013) fuel efficiency

This year our mileage was higher primarily due to taking a ~1000 mile road trip in western Nebraska and Colorado. Ingrid also started driving the Prius more and her own (now sold) car less on evenings and weekends.

Just like I wrote in 2015, I believe main thing lowering the fuel economy is short winter drives. My drive to and from work is just 2.5 miles per direction, and I often drive a similar distance to and from lunch. In winter, that's too little distance for the car to warm up to the point that it enters the fuel-saving EV mode while stopped at lights.

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Will an Electric Vehicle save on CO₂ emissions anyway?

tl;dr: Compared to a hybrid like the Toyota Prius, the incremental emissions for a Tesla using typical US electricity sources are higher. They just don't occur at the car's tailpipe.

After recently asking whether a person with my habits could ever save money driving a Tesla instead of a Prius, I also wondered about per-mile emissions. Others have investigated the extra costs of EV manufacture and disposal (many elements of which apply to a hybrid as well), while my research is only about the incremental CO₂ emissions created by whatever energy inputs the vehicle needs.

Here's the spreadsheet itself, which includes links to sources for each number.

The top result is that the source of electricity makes a huge difference: China (major electricity source: coal) has 9x the emissions per kWh than France (major electricity source: nuclear). Canada has under 50% the emissions of the US, and I was happy to see that my local electrical utility is 10% under the US national average.

Using EPA fuel economy and the 2015 US mix, I calculate 47.7 lb CO₂/100 miles in the Tesla, and 40.0 in the Prius.

Of course, if you're comparing the Tesla to an "average" US car, it's a different story: The 2014 fleet economy for passenger cars is 31.5MPG, leading to emissions of 71.11 lb CO₂/100 miles.

If you're living in a country that has more nuclear or "renewable" energy, like France, you'll also come out ahead: with only 0.26 lb CO₂/kWh in 2008, a Tesla would put only 21% as much CO₂ into the atmosphere as a Prius running on gasoline.

I also took a stab at how Clinton's clean energy plan would change the numbers. My first guess is that it woud lower the lb CO₂/kWh by around 1/3, which still doesn't make us as clean as Canada's 2008 mix.

Finally, I checked how a Prius would do running on E85, assuming that carbon-neutral ethanol production were actually possible. This brings the prius down to 8.34 lb CO₂/100mi, within spitting distance of the cleanest electricity of countries I surveyed, 8.21 lb CO₂/100mi running a Tesla off the French grid.

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Does an electric car make economic sense?

I drive 8000 miles a year in a car with 45MPG actual fuel efficiency (2013 Prius). We paid somewhere around $23000 for it. If I drive this car for 15 years, I'll buy around 2700 gallons of gas.

Compare this to the (discontinued) Prius Plugin Hybrid with MSRP of about $30000. Imagine that I could have done all my driving in electric mode, and that its efficiency is ∞MPGe. I'd break even on the $7000 higher initial price if the average gas price is $2.56. Sounds plausible that I could save money that way, right?

But of course I couldn't go everywhere in "all-electric" / "charge depletion" mode, probably only about 2000 miles/year out of my 8000 miles/year would fall into this category (in-city driving 200 days a year at 10 miles/day). So now I'm only saving only about 670 gallons of gas *OVER THE COURSE OF OWNING THE CAR FOR 15 YEARS*. This is only break-even if gas is $10/gallon.

But of course, the plug-in hybrid is not ∞MPGe, it's 110MPGe. MPGe is actually "miles per 33.7kWh". 33.7kWh of electricity costs $3.17 in my local market at summer rates, or $1.88 at winter rates, an average of $2.31 (only 4 months are "summer"). So that's $623 in electricity to operate the thing in electric mode for 30,000 miles. This pushes the break-even point higher, to around $11/gal gasoline. (Note: a previous version of this page used incorrect math to arrive at a figure of $19 as the break-even point)

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One year in a Prius

For everyone not still suffering under the tyrrany of imperial measures, 44.4mpg is around 5.3l/100km and the distance driven is about 12000km.

That means ...

• 260 hours total driving time
• 43 minutes driving time per day
• 21 average miles a day (33km)
• 170 gallons of fuel (640l)
• 2/3 gallon per hour of driving (2.4l/hour)

Getting at 44.4mpg instead of the EPA combined fuel economy of 50mpg means using 20 extra gallons of fuel over the course of the year, 13% more than predicted.

Overall we're happy with the Prius. It's much roomier than the car it replaced and it has better fuel economy. However, I'm a bit sad we didn't get closer to the EPA fuel economy.

The main thing lowering the fuel economy is short winter drives. My drive to and from work is just 2.5 miles per direction, and I often drive a similar distance to and from lunch. In winter, that's too little distance for the car to warm up to the point that it enters the fuel-saving EV mode while stopped at lights. (this kind of driving is even classified as "severe operating conditions" by Toyota: "Repeated trips of less than 5 miles in temperatures below 32 degrees Fahrenheit")

Our summertime fuel economy (also the time of year when we make long driving trips on the highway) is closer to 48mpg.

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