Logan’s recent post reminded me to highlight why MPG is a terrible measure that is severely distorting consumer and regulatory behavior.
If you had to guess, which do you think saves more gas: going from 20 MPG to 30 MPG, or 30 MPG to 45 MPG? Well the latter is an increase of 15 MPG so you might be tempted to choose that, but they are both 50% increases, so maybe they are the same, right?
Well let me add in more detail. Let’s say you are driving 100 miles. Now calculate how much fuel each would take:
20 MPG = 5 gallons
30 MPG = 3.33 gallons
45 MPG = 2.22 gallons
Well now it’s clear, going from 20-30 saves 1.66 gallons while going from 30-45 only saves 1.11. Of course we can expand it even further, since a lot of smaller cars now get around 40 mpg while even the best hybrids only get 60 mpg. That is saving a mere 0.83 gallons/100 miles. So why do we look at it in terms of MPG instead of a more realistic measure? I actually have no idea; I’m genuinely asking since apparently if you walk into a dealership in any country in the world except the US, they will display efficiency in terms of litres per 100 km so you can make accurate comparisons.
Let’s do a comparison for an average midsized car that gets 30 mpg to an average hybrid that gets 45 mpg. In general the hybrid costs about $6k more, which at a price of $3 per gallon (we’ll assume price will continue to go up) is 2000 gallons. Since you are saving 1.11 gallons per 100 miles, that means you have to drive around 180,000 miles in order to break even in terms of price. OK you say, let’s say price isn’t the primary purpose, saving energy is. Well it is hard to find good sources about how much energy goes into hybrid batteries, but based on their price you can estimate that it’s about 900 gallons (I used $3k of electricity for the battery) or break even at 80,000 miles when compared to a 30 MPG car. This means that you are saving about 1100 of gas in terms of energy.
Now let’s look at the 20 MPG to 30 MPG. The 180,000 miles it would take to break even in cost above would result in $9k of cost savings for 3000 gallons of gas. Obviously targeting the ~20 MPG of the market and moving that up to 30-35 MPG is far more important for both pocketbooks and the environment than having people that currently get ~30 move into hybrids, yet in practice that is not what is happening.
How realistic is it to move the average MPG (around 24) to around 35 MPG? I think it’s very realistic with existing technology if we move to diesel engines which get about 40% more MPG. These aren’t your father’s diesel engines either, since with modern computer controlled systems most of the drawbacks of diesel are addressed. This is why 50% of all cars and nearly all sedans in Europe are now diesel, leading to an average of 40 mpg!
Making this transition will take a lot of changes in marketing for consumers and also regulatory agencies that define things in terms of MPG. In fact a lot of designs for passenger diesel cars are getting scrapped because emission regulations are per gallon instead of per mile, and diesel engines cannot pass them even though diesel blows away gas engines in terms of emissions per mile, also known as reality. Moving to gallons per 100 miles in both marketing and regulations would allow market forces to create better cars (or just have the same design here that they do in Europe) based on increased consumer demand.
It’s much more simple and impactful than fixation on hybrids.