Lamenting Miles per Gallon
I brought this up once before, but with all the recent griping over the Cash for Clunkers Program and its lack luster milage requirements, it’s worth reflecting on how confusing miles per gallon ratings are from the perspective of trying to conserve gasoline. What we should be talking about is gallons per mile, (or perhaps gallons per 10 miles, or decamile, just so the numbers are nicer).
The Cash for Clunkers Program offers you a of $3500 rebate if you trade in a car that gets at most 18 miles per gallon, and purchase a car that gets at least 22 miles per gallon (and of course you can’t just use your rebate to buy a 5 year supply of metro cards, because I guess that would save too much gas). 22 MPGs strikes many as a pretty meager milage requirement (because it is), which is probably why the program offers an additional $1000 if your newly purchased vehicle increases your fuel milage by 10 MPGs (although both rebate conditions are less strict for SUVs and trucks).
Encouragingly, preliminary reports seem to indicate that the majority of trade-ins are meeting the 10 MPG requirement. Still, one can’t help but wonder if a minimum fuel requirement closer to 30 MPGs would make the prograe more worthwhile (particularly since today’s new cars are required, on average, to get 27.5 MPGs, and this number will increase to 30 in 2011). Perhaps it would, but the current situation isn’t as dire as you might think.
Consider the following example:
You trade in a car that gets 17 miles per gallon. That’s quite close to .59 gallons per 10 miles. You switch to a 24 mile per gallon vehicle, which is a bit under .42 gallons per 10 miles. Assuming you drive 1200 miles per month, and don’t change your driving habits, you’ve saved over .17 x 1250 = 20.4 gallons of gas. Now suppose you wanted to save an additional 20 gallons of gas, you’d need to go all the way to 40 miles per gallon. At that point you’d be down to 30 gallons of gas a month, and saving the next 20 gallons would require that your car’s milage enter into hypothetical chevy volt territory.
Of course the above example isn’t completely honest. People do drive about 1200 miles per month, but people also change their driving habits based on their car’s fuel consumption. My intuition is that this only leads to a modest reduction in fuel savings (particularly when gas is expensive), but it’d be interesting to see some real numbers.
Incidentally, in Europe, where conservation is all the rage, they already do the whole gallons per mile thing (except they call it liters per kilometer, which is sooooo like them). In contrast, miles per gallon strikes me as a reflection of this country’s love affair with the open road. Here in the states, when we’re looking to buy a car, we want to fill that bad boy up with a tank of gas and drive as far away as humanly possible. In Europe I’m pretty sure they just all sit around by fountains or something.
September 21st, 2009 at 2:56 pm
In aviation we express it in mass flow amounts or as a volumetric per hour as in General Aviation (GPH gallons per hour) I don’t know if that would help people in grasping their usage but I would say not. We rode bikes mostly in Germany and trains. Some of us had 100cc motorrads for trips to the lake but who needed to drive to go somewhere fun or cool or get what we needed? Maybe things were simpler for us and the distance was less than here in the US.
BTW I ride an electric bicycle to work with a hub motor. I am planning a carbon fibre recumbent with a hub motor next. Less drag more volts same motor I should be moving pretty good.
September 28th, 2009 at 8:18 pm
Agreed. I think, though, that no matter how you slice it, every little bit counts. Meaning we need people to invest in these upper echelon fuel saving vehicles (Chevy Volts, Priuses, etc.) so we move the market in that direction. If we do not demand it, they will stop supplying it. Or stop trying to supply it in innovative, nichey ways. I still see a lot of people driving big big cars. While I realize that the jump from a 17 mpg to a 22 is more significant than a 22 to a 30, I still think, in terms of momentum, we will get where we need to be environmentally, if we still push for higher standards.