In “Good Math, Bad Math,” Ph.D. carrying, Google-employed, shmartypants Mark Chu-Carroll talks about how math can be used, incorrectly, to mislead people into having expectations that do not match up to reality.
His latest post points out a particularly interesting one – the bad math used to calculate the mileage on the Chevy Volt.
The Volt, if you’re not familiar with it, is a type of vehicle called a “plug-in” hybrid. In a normal hybrid, the internal combustion engine powers a generator which charges batteries, and the car runs off of the electric juice in those batteries. This generally produces better gas mileage than standard internal combustion engines – basically because the batteries in hybrids can store energy that is normally “lost” in internal combustion engines – specifically, in braking and idling.
So anyway, the Chevy Volt simply takes the next step; it not only takes electric power from the onboard internal combustion engine, but also allows you to draw electricity in off-peak hours from the municipal grid. You literally plug this sucker into an outlet outside your house.
Now this isn’t particularly new technology either. Hobbyists and car enthusiasts have been creating “PHEVs” (Plug-in Hybrid Electric Vehicles) since 2004. This is the first car, however, that has plug-in as a default, off the factory rack, no voiding the warranty needed.
But because of the technology needed, it was always hard to figure out exactly how fuel-efficient plug-in-hybrids were. Hybrid cars – those cars that get all of their electricity from the internal combustion engine – had rather simpler metrics. But plug-in hybrids use zero gallons of gas for the first few dozen miles – for the Chevy Volt, that’s about 40 miles without a need for gas. But once you get over that limit and start burning gas, the volt is no more fuel efficient than a normal hybrid – say, 50 miles per gallon.
So depending how far you drive between 8kwh charge cycles, you’re looking at anywhere from slightly above 50 mpg to “infinity” Mpg. Where’d this 230 number come from?
Well, the U.S. Environmental Protection Agency drafted some guidelines for coming up with an MPG figure – with, of course, some “guidance” from the auto industry. Of course, this is a literal case of “Your Mileage May Vary.” Chevy is claiming the 230 mpg number based on its own internal tests, however, as the EPA has not yet tested a Volt.
You may argue that a more accurate nomenclature, say “50mpg + 40” is confusing to consumers who are used to thinking in cars in terms of “miles per gallon” – but you know what’s going to be even more confusing? When they find out that their 230 mpg car might get as little as 50 mpg.
One of the big problems with trying to figure out something that’s “per” anything else – is that essentially, our brains aren’t wired for division. They’re wired for addition, subtraction, and multiplication. It’s counterintuitive, but true, that a hummer that improves its mileage from 10mpg to 15mpg saves more gas (about 3.33gal/100 miles) than a hybrid going from 40mpg to 50mpg. (about 0.5gal/100 miles). I mean, I’d still get the hybrid over the hummer, but you see where I’m going with this.
Of course, we’re no strangers to bad math ourselves in IT. Bad math is one of the reasons that you need to independently confirm service level agreements with various bandwidth providers. They may promise something like “100ms” latency, but there’s a big difference between a maximum of 100ms latency and an average of 100ms latency. Network visibility isn’t just looking at the numbers, but knowing what the numbers mean.
