A few weeks ago I posted a critique of CNW’s lifetime energy cost report, which controversially concluded that a Prius uses up more energy per mile over its lifetime than a Hummer. My critique then was based simply on the ridiculously high cost per mile figures the report included. From this I inferred that some untenable assumptions were made by CNW, but I didn’t read deeply enough to discover these assumptions.
Then I made the mistake of stepping into a thread about this study over at elcova, an active Hummer forum. The resulting discussion forced me to dig a little deeper, unearthing some of the beyond shaky assumptions made by CNW.
To begin with, CNW assumed that a Prius would last only 100,000 miles, supposedly because this is what Toyota claimed. I can see this figure applying to the battery pack, but not the entire car.
Next, CNW divided the costs of developing the technology and the car itself over the number of cars produced to date. A more reasonable assumption would divide these costs over expected future as well as past production.
Similarly, CNW divided the costs of building a factory over the cars it has produced to date, not future as well as past production.
These assumptions are highly unrealistic, and bias the results in favor of old car designs that use old technology. Even with them, though, I ‘m not sure how they arrived at such high cost per mile figures.