Visualizing the Evolution of Cars

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From user 27147 on Flickr, Creative Commons licensed, some rights reserved.

I've always been a big fan of cars. Like with sports, cars are a subject for which people love to quantify things. Horsepower, curb weight, engine displacement, top speed, miles per gallon... all of these attributes are carefully measured, graded, evaluated, and play a factor in the car market. In high school I knew every stat for my '87 D50...

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

Getting Bigger
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:
Source article

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 Stronger

Looking 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 Better

While 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.
Source article

This got me excited about the actual improvement in cars, beyond the addition of bluetooth syncing and spinners. So using the large database on cars from fueleconomy.gov, I investigated the improvements of cars since the 80's.

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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Data Sources

Some similar analysis

5 comments:

  1. Have you corrected for the changes in how horsepower is reported? As I understand it, until 1972 the US used SAE gross horsepower, which drastically overestimated BHP. The Wikipedia page for the old Charger (not necessarily a representative car, and not necessarily the best source for engine figures) indicates that the same engine went from 350 to 280 reported HP because of the change.

    ReplyDelete
    Replies
    1. I haven't applied any correction for this change. I am wondering how the NHTSA historical data was calculated, but I couldn't find any information on their website about it. As always, there's a major "caveat emptor" when using publicly available data...

      Still, the drop from the late 60's to late 70's is big enough that it's definitely telling a story!

      Delete
    2. Yeah, I came to ask the same thing. Around the same time was a pretty big hit to power that came from government mandated emissions equipment that US manufacturers weren't really prepared for.

      Delete
  2. I wonder how many pound-miles cars carry, on average, now versus the historical periods studies above?

    ReplyDelete
  3. Very interesting analysis. Could you please post your code you have used.

    ReplyDelete

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