August Fuel Economy Update - Impressive Range - 2016 Toyota Prius Long-Term Road Test

Edmunds.com

August Fuel Economy Update - Impressive Range - 2016 Toyota Prius Long-Term Road Test

We're still shy of 5,000 miles in our long-term 2016 Toyota Prius, but we're not yet seeing the 50+ mpg that gave the new Prius so much promise.

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kirkhilles1

Yeah, sub-50 seems low for a quality hybrid. I mean, you can get 35 mpg all day long in the Civic and 46 easily on the Highway, so you guys should be hitting low 50s even in LA Traffic.

actualsize

Note that MPG is a non-linear unit. 1 mpg has no fixed value. This is because the the dependent variable - gallons of fuel burned - is on the wrong side of the fraction. Here's one example of this non-linearity: 5 mpg off of a 50-mpg rating is 10 percent (I'm using 50 instead of 52 to make the math easier), just as 2 mpg off of the 20 mpg rating of a pickup is also 10 percent. Both 45 versus 50 and 18 versus 20 are 10-percent misses, but at first glance the 5 mpg miss LOOKS worse than the 2 mpg miss. It isn't.

In fact, that 5 mpg miss on our Prius is actually a much SMALLER deal that the 2 mpg miss would be on a pickup. How can that be?

Invert 50 mpg you get 0.2 gallons per mile. Multiply both sides of the fraction by 100 to get rid of the decimal and put the number in a more familiar and easily digested range and the same value amounts to 2 gallons per 100 miles, as in the 50-miles-per-gallon Prius burns 2 gallons of gasoline during a 100-mile trip. Easy. Now do the same to 45 mpg and you get 2.22 gallons per 100 miles. So now we can see that our 5 mpg miss (45 mpg instead of 50 mpg) represents an extra 0.22 gallons of fuel burned over 100 miles.

Now look at 20 mpg and 18 mpg, our pickup truck's similar-sized 10-percent miss that looks less significant because it's "only" 2 mpg. Invert 20 mpg and you get 5 gallons per 100 miles. Now invert 18 mpg, and you get 5.55 gallons per 100 miles. That 2 mpg miss represents an extra 0.55 gallons of fuel burned every 100 miles. This 2-mpg miss will cost you more (and represents more unexpected emissions) than the Prius' 5-mpg miss.

45 mpg versus an EPA rating of 50 mpg IS LESS THAN HALF AS SIGNIFICANT than 18 mpg is compared to a rating of 20 mpg.

5 mpg is less than 2 mpg, in other words, and I can only say that because MPG itself is a mathematically bankrupt way to make such comparisons. MPG is a stupid unit, but gallons per mile is perfectly fine because the dependent variable - gallons burned -- is on the top of the fraction where it belongs, not the bottom.

The moral: don't get too upset because the number is 5. It's a bad unit. Would it be better if it were zero? Sure. But the magnitude of 5 here is meaningless. 5 what? MPG isn't a tangible thing. But everything makes a lot more sense and comparisons are much easier to make if you convert everything to gallons burned every 100 miles.

Going back to the fuel economy summary data, our 2016 Prius has burned an average of 2.1 gallons every 100 miles instead of its EPA combined rating of 1.9 gallons per 100 miles.

As the EPA rating gets bigger, the value of 1 mpg gets smaller AND LESS SIGNIFICANT. Say a car is rated at 100 mpg but only gets 90 mpg. OMG! That's a 10 mpg miss! But its the same 10-percent miss of our other examples. 100 mpg is 1.0 gallons per 100 miles, and 90 mpg is 1.1 gallons per 100 miles. We're talking 0.1 gallons every 100 miles. That's only 1 extra gallon in a 1,000-mile month of driving. Meanwhile, if your 20-mpg pickup only got 10 mpg, that'd 10 be 50 extra gallons in a 1,000-mile moth of driving. 10 mpg does not equal 10 mpg!!!

gauchograd99

I would love to see the data on one of these from someone who lives and constantly drives around town in a hilly town like Lake Forest, Mission Viejo, La Cañada, etc. I am always wondering how well the uphill vs downhill in a hybrid averages out. Seems to work out well with a small turbocharged engine, so I would assume a hybrid would also average out in the end.