I’ve been doing some research into car fuel economy and how easy it would be to increase the efficiency of cars. Two things I needed to find were how long cars last and how many miles they are driven. I found an interesting Department of Transportation study with equations for this information. Car lifetimes are expressed as percent surviving by age. There is initially a slow decline in the number of cars from a modelyear on the road, then a quick drop:

Car survivability by age

The number of miles driven in a year decreases with the age of a car:

Annual distance traveled by car age

I combined these two equations to find the percentage of all miles driven in a year for cars by age. The combination of these two factors decreases more rapidly than either alone:

Percent of all miles by car age

Even in the period where most cars for a model year are on the road, the number of miles the older cars are driven decreases. This means the average fuel economy for all cars on the road has very little hysteresis for changes in the average economy of new cars. Basically, the average fuel economy of all cars on the road will be close to the fuel economy of the current model year. The numbers for light trucks (all pickups and most SUVs) are similar. This means that any changes to fuel economy standards, or the popularity of SUVs versus cars, will quickly change the total gasoline consumption for the country.

UPDATE (20090129): What this directly shows is the percent of total distance for a model year driven by car age. If the same number of cars are built each year, the results also work for all cars on the road. If fewer cars than average are built, the delay will be a little longer. If more are built, the delay will be shorter.