Opinions differ on why average fuel economy hasn’t changed in 20 years (industry inertia, market demand, greed, conspiracy, physics…), but the facts are clear – for example, as documented in a newly-released EPA report: “Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through 2006". This graphic (from coverage in the Washington Post) says it all:
Average miles per gallon, cars and light trucks, by model year
SOURCE: Environmental Protection Agency | GRAPHIC: Karen Yourish, The Washington Post - July 18, 2006
The problem isn’t a lack of engine technology advances. It’s that the advances haven’t been applied to increasing miles-per-gallon. Another graphic that says it all (this one from the Executive Summary of the EPA Report):
Note that the average weight drop during 1975-1981 corresponded to an increase in fuel-efficiency (earlier graphic). But weight, while hugely important, isn’t the only factor. This is clear since MPG remained constant after 1981 despite the increase in weight and acceleration.
It makes you wonder…. What average MPG would we have today if weight and acceleration time had remained constant after 1981?
Here's a (first order) conjecture based on extrapolating the EPA data: Given that the average MPG increased roughly 7 MPG during 1975-1981, if that rate of change had continued (with constant average weight and acceleration), the average MPG today would be 42 MPG rather than 21 MPG.