The reason for the increase is a change in the model mix. As sales of many larger trucks and SUVs have sagged, many car models have made gains. After a dip late in 2006, gas prices have stayed relatively high through most of this year. Another factor is the sliding housing market which has cut into full-size pickup sales as construction starts have tanked. As long as fuel prices remain high, the trend is likely to continue.
[Source: Detroit Free Press, thanks to Justin for the tip]