|From user 27147 on Flickr, Creative Commons licensed, some rights reserved.|
When I looked at cars over the last couple decades, the change in one particular attribute stood out to me: cars are getting big! (both heavier and larger) I'm not the only one noticing this, of course, and cars used to be very large. So I asked myself: How have car sizes, and efficiencies, changed with time?
I started by looking at data for one of my all time favorite brands: BMW
Here is the curb weight for the BMW 3-series, M3, and 5-series over their entire history. Also shown: the average passenger car from the National Highway Traffic Safety Administration (NHTSA) since the 1950's.
A few things stand out to me: The 3-series seems to follow the typical car trend for most of its life. Also, check out that huge drop in curb weight in the 1970's, due largely to the oil crisis.
Sure enough, the 3-series (and every other car) has been packing on the pounds over the last 20 years. Here's the same kind of diagram for the Honda Accord, also showing the evolution in horsepower:
Here are both BHP and curb weight since the 1950's. While both curves bottom out at around 1980, the horsepower starts to drop off as early as 1970, while the curb weight takes another 6 or 7 years to drop. The impression this gives me: people were willing to sacrifice power before they gave up luxury,
Getting StrongerLooking at the horsepower evolution more over time, here I compare the BMW 3-series horsepower to that of the normal car sold in America (again from NHTSA). In this figure I also have the BPA data for median passenger car, which yields a larger estimate for the typical horsepower since ~1999.
For the beginning of its life, the 3-series was a typical sized car with a powerful engine. Mixed with the luxurious interior, this truly was the "Ultimate Driving Machine". Sometime around 1999, however, the BMW became downright average according to this figure. I've heard similar sentiments from people, underwhelmed by the sportiness of the late 90's / early 00's 3-series. After 2010 BMW got back on the "horse", so to speak, and the latest incarnation of 3-series vehicles are absolute beasts!
Getting BetterWhile cars (and Americans) have been getting bigger for the last 20 years, cars have at least become more efficient... the jury's still out about people. Here is a figure showing the slight increase in fuel economy, despite the increase in curb weight, from 1980 to 2006.
Here is a grid of the fuel efficiency for 33,058 cars since the mid 1980's till today. The improvement in MPG has been marginal.
Here now is the correlation between MPG and engine displacement for the same 33k cars. As you expect, the cars with tiny engines get great milage, and the monsters with 6L engines get terrible MPG. The orange line is a power law fit.
Finally, I decided to break this down by year, fitting power laws for cars from each year's data. This produced an awesome result: Cars really are getting more efficient! Despite getting heavier and more feature rich, as well as steadily increasing horsepower, passenger vehicles are getting more efficient!
There's a wealth of information kept about these 33k cars, including the evolution of your favorite brand or model! I'd love to see more analysis of this data. I also have become fascinated with the notion of a principle component analysis (or the like) of the body styles of cars over time, hopefully to quantify the evolution of styles! If anyone has a huge database of clean car photos, I think this would be a super cool project...
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Some similar analysis