Gas mileage

Our friend Paul is traveling the country on his Honda Shadow, and is apparently getting a disappointing 30 or so miles to the gallon (usually he gets 50+). Real world factors cutting the milage of his v-twin 750 in half include: ethanol, wind, and speed. In the less populous states, people move along the highway much faster, which is bad for milage. 

This got me thinking about my own gas mpg, as Claire and I own three very different cars. Our truck, a 3.0 litre V6 gets about 20 mpg. It's a big truck, but it's a stick shift and the vulcan v6 has a knock sensor so it runs well on regular. My echo has taken a dive over the last few years, from 37 or so down to 30-33. It may be time for a new air filter. The prius does much better; better than most people think.

If you own a Prius, you might feel a little let down by the average gas milage the readout calculates for you automatically. The problem with this is that it discourages you from calculating your own gas milage at the pump by dividing, which is a shame because the prius (like the echo) gets about 10 gallons for a full tank, making the calculation really easy. If you pump 10 gallons, you can just drop a digit off how far you went on the tank (if you pumped 5 gallons, double it first). The last time I did this with the prius we'd gone about 500 miles on about 10 gallons, so 50 mpg. The readout said 46mpg. I didn't get why at the time, but I logged this information away. 

The other day I finally figured out why when we were tooling around, and I coasted to a stoplight to charge up the batteries. When I was in school, ( http://cs.gmu.edu/~eclab/projects/robots/flockbots/pmwiki.php?n=Main.AuthorList ) we built some robots senior year. One of the problems we had to learn was that if you try to calculate an instantaneous speed off of a digital encoder, it's either 0 or infinity. You have to count the readings of infinity over a time period. The Prius faces a similar problem calculating instantaneous gas milage, and they stretch out the calculation over a few seconds to combine fuel flow with velocity. When you take your foot off the gas, your instantaneous gas milage actually approaches infinity - you know, dividing by zero! Since the fine engineers at Toyota would like you to see a nice clean graph of your instantaneous gas milage, they artificially cut off the graph at 100 mpg. 

My theory is that they can't actually capture the information that you're getting infinity mpg during a coast or when braking, and this pulls the average down from reality. The more coasting, the further off the reading. Toyota has probably thought about this a lot, and concluded that it wouldn't make sense to display the infinity symbol on the graph, and the computer couldn't calculate it anyways. I have a solution however, if they displayed the gas usage/hour "instantaneously" by looking at a few seconds at a time, they could avoid showing an asymptotic value, because the amount of fuel flowing even at a very slow speed will be finite. The fuel rate gauge would only go to a maximum value if you were trying to go, and were stuck somehow, which happens so rarely that you could view it as a non-issue. 

Unfortunately, the Toyota engineers probably discarded this idea too, because people only care about mpg. Maybe it would have been better to leave out the reading, so people wouldn't feel slighted.