The U.S. economy is growing limply, as it has for years. If it weren't for trucks, we might scarcely be growing at all.

The average household spent $51,400 in the last year—about $800 more than the previous 12 months—and higher spending in one sector made up most of the difference. It wasn't our anxious health care industry, or our enormous housing market, or even necessities like food and clothing. It was higher spending on transportation—mostly on new and used vehicles—that accounted for 60 percent of all additional dollars spent by households over the last year, according to the Bureau of Labor Statistics.

Here's a look at where the higher average household spending came from in the 12 months before July 2013 (compared with the 12 months before July 2012). It's pretty much all transportation and cash contributions, the latter a catch-all category that notably includes financial support for college students.

New and used vehicle sales are on fire. But domestic car sales are down 6 percent. How that can make any sense?

It turns that Americans are over cars. We're buying trucks, instead.

Domestic and imported truck sales are up 8 percent in 2014, and they account for more than 100 percent of all new vehicle purchases, off-setting the decline in new cars. (The used-vehicle market seems to skew slightly toward smaller durable cars, but it's hard to find great data.) Some smaller models, like the Toyota Camry and Hyundai Sonata, are outselling last year. But overall, bigger is selling better across the board.